And yes, Conan, your initial answer was, at least, condescending and judgemental. The follow-up didn't even get better. On the other hand, who could expect good manners from a paleolithic criminal?

You win the internet for your insightful takes on my username. Well done- it made me smile. Thanks.

Alas, you didn't take the bait.

Isn't your response to my response a definitive example of one person passing judgement on another?

How about this alternative? At the end of the day, we want to follow the goals and requests offered by the OP. I list these ideas as:

A definitive Dark Side user among a party of other Light Side users...

wants a small quest to get a more powerful lightsaber, which requires a gem...

...has a Sith-like artifact that seems to function like a holocron (my opinion of functionality, but props on NOT just naming it "Dark Side Holocron")

but the character realizes the Dark Side-influenced actions, and has remorse, perhaps returning to the light Side.

Add to this mix, an 8-10 year old girl witness to the jewel theft is offered as an easy sacrifice to fall into the Dark Side.

Here, killing the kid is too easy- we expect that to happen, and it locks in the character to a (possibly strong-armed) decision.

If there is this super-awesome jewel that makes really cool lightsabers, we might assume more than one person has placed value on it, too, if only monetary. Cue the kid. Let's envision she's just a street thief, and is curious about someone else trying to steal the jewel she wants to steal. She'll be well skilled in running away, stealth, and maybe generic thieving skills (from acrobatics to skulduggery). When the jewel is stolen, she appears with a good perception check. We assume the Dark Side user wants no witnesses, as commanded by the holocron-y.

Start a chase. Holding the jewel, run through back allies or other terrain. Into this race back to her keepers and other thug-kids, drop at least one security presence, preferably one with ranged energy weapons. Make the PC do some cool dice rolls in which they will likely succeed, like maybe acrobatics over carts or crates, conditioning to lift themselves over a wall, use Move Force power on a slowly opening door in the alley, etc. Let the player feel good about their rolls, as well as storytelling. Players like rolling dice, so let 'em.

Have the security guard/droid miss the PC terribly; one shot will work (or, better yet if using Threats/Despair on any checks). The energy shot ricochets off of the jewel and strikes the little girl...just as she drops dead at her keeper's feet at an orphanage. Even if the blaster were set to stun, the reflection off the jewel did the needed damage.

The PC still has to eliminate the security guard.droid as a witness. As they finish that task, the Dark Side user turn to see a bunch of open-jawed kids stepping out from a Chaz Dickens' novel.

Sensing death and destruction, the holocron-y reawakens to see the girl witness he ordered killed is, in fact, dead - this Dark Side user will get whatever benefit is sought about building an awesome 'saber because of their supposed super bad evil actions.

...but the PC is now faced with an orphanage of witnesses... almost like at the Jedi Temple with newly christened Dark Vader, huh ? Will your PC take the bait and recreate, or will this be enough of an evocative image to hit home that message of redemption? Have the kids seem to be in very bad shape, asking for help in which the PC could assist. If the party needs another adventure hook or small allies in the streets, you now have 'em...

...without having the player kill a kid. Well, I guess they still could, but again, if you or they were serious about redemption, I offer you a stark, direct image drawn from canon about just how Dark the Dark Side can get. Will your player opt out at that point? If they really want to go all in, I also offer you a way to fully embrace the dark game you want to tell- kill 'em all.

How 'dat?

I'll just share what I posted to the OP in a PM exchange:

...it sounds like you have a lot of good ideas in there. Mother and Father issues are always good for a redemption story. The only problem I see is trying to cram it all into one session. I would try to split it across at least 3 because you need time to get around the amnesia thing. That way in the first session you can introduce something he needs to start caring about, second session more memories come back and the caring is reinforced, and third session brings clarity and an awful choice. The original Bourne trilogy might be an interesting parallel...he has amnesia because he wanted to forget that he volunteered. In that case, the whole road to redemption actually starts with Bourne not acting because of the child. Then he is shot and left for dead. So in some ways what you thought of as the end could actually just be the beginning.

I'm just spitballing here, but with Bourne in mind, maybe some aspects of his sessions are flashbacks. If you can weave some encounters in seamlessly that end with him "waking up", it might be a way to handle the amnesia. Maybe the choice with the child is already in his past (he didn't do it), but he keeps reliving the moment ... a form of PTSD where he's constantly afraid of making the wrong decision, but that torment just drives him to do the dark things he's been doing to the other people in the party. Maybe he doesn't realize he already made the right decision, or he's just afraid of being presented with that kind of choice again.

You can also weave in Mother/Father stuff...maybe one or both were a real evil piece of work. Maybe one actually made him kill the other (hence the amnesia), or similar to above, maybe he didn't and ran, but keeps reliving the choice.

As for penalties, I'm kind of a fan of the maiming thing, just because it seems to be a standard Star Wars trope that maiming can lead both to despair and clarity. If the price of turning back to the light is that he is so hideous to look upon that any Charm attempts always have upgraded difficulty and a couple setback...he can always take talents like "Kill with Kindness" to remove the setback, but the upgrade will be there forever.

I've skipped a few posts... sorry if I'm repeating somebody else's idea...

I know it's a plot used so many times if fiction it would be pretty weak... but it's also so effective

Have your player trapped by a Dark Side Master that ask him to sacrifice someone before he can be released... so the Dark Side Master captured 2 others persons... maybe an unknown child, smart and in good health.... and an old friend, maybe old or sick, but with some personnal history with the character.... now the Dark Side Master ask the player to kill one of them in cold blood and that he will be released with the other survivor after the deed.... If the player refuses to kill one of them, then the Dark Side Master tells him he will kill both before his eyes and then kill him after.

Once the player kills one of the 2 victims, the Dark Side Master kills the other victim.... if the player tries to intervene, the Dark Side Master stuns him with the Force (Bind) and kills the other anyway before him.... he then tells the player to feed from the Anger and Rage that fills him... now maybe the player will strike down the master that just disappears like Obi-Wan did in Ep4.

Give him 2 bad choices... both Dark...

Hope it helps

Besides that you never tell what a PC decides, I have a much bigger problem.

Your planned terrible, terrible act is a sunday morning raid by the rebels. The good guys kill all the time, they kill civilians, mothers, fathers, children. That is part of their daily life and the simple fact of guerrilla war.

If your player WANTS to do something terrible then you need to give him the option not to kill that girl. The more senseless and pointless her death the better. And if you want to make something like this the break point for a character, the point when he either never can return or watch at his past action in disgust and tries to change his way then you have to slowly poison him. Let him kill for convenience, let him kill stormtroopers without second thought, drive him into countless situations when murder seems like a good choice until you influence not only the character, but as well the player as well to assume that killing to achieve his goals is ok.

It will be easy to make him kill civilians at this point, children or not, just business as usual, Just another day of war. Nothing terrible happening until you think about it for a moment.

A million dead civilians in war against a few thousand terrorists, welcome to the empire, you are now ready to join and restore order and justice with iron fist. ;-)

Uhm, wait, I got off-topic. But you get the drift. The easiest way to get a character or a group of characters do terrible things is to make it appear normal. Let him torture to save people first, maim and mutilate the enemy to get informations, make it appear effective, but remember that actually torture rarely gives any reliable informations.

It super easy for example to let npcs act cruel first and be successful with their methods and beneficial for the characters. Use all those torture movie tropes, and goals justify all means tropes, etc

Oh and if you really have to much time, just watch "the clone wars, season 2 to 5 should be enough to get an idea how easy a righteous man can fall when a war is going on. But I admit, it a lot of stuff to catch up.

And yes, Conan, your initial answer was, at least, condescending and judgemental. The follow-up didn't even get better. On the other hand, who could expect good manners from a paleolithic criminal?

You win the internet for your insightful takes on my username. Well done- it made me smile. Thanks.

Alas, you didn't take the bait.

Isn't your response to my response a definitive example of one person passing judgement on another?

Everybody is judging everybody else all the time, based on their, most often very limited, information about them.

In fact, the first post was intended to provoke a reaction (of any kind), and it did. A not so prejudiced anymore, but a little more informed judgement was to be passed afterwards, and it was.

The response itself was a tip o'the hat to your response.

In so many words: Indeed, it is, but a positive one.

Oh, and, sorry for actually answering a supposedly rhetorical question! I just can't help it. Feel free to judge me, based on this!

Give him the "sadistic choice". Find a way to set it up that he has to kill the girl to save someone else. Falling to the dark side usually starts where the ends justify the means. Maybe he knows that if any sound escapes the camp, some beast will attack, killing everyone. He has to stop the girl from screaming, and the only sure way is to kill her.

*Update* I ran the one on one session last weekend and it went over well. I decided to leave the identity of the kid a secret and only say that there was a small figure in a concealing hooded cloak that went to nab the crystal as soon as he got the protective glass case off of it. I made sure to notate the features that it doesn't look like a full size human adult, but at the same time I didn't overdo it so he might have thought it was an alien that was short.

Either way once the NPC grabbed it and ran off so a chase started. Before either of them got very far the PC took out his blaster and shot her in the back with a very solid hit. She had next to no soak and very little in terms of stats so she went down right away. Once he went over and took back the crystal he saw how old she was and immediately felt bad that he didn't even think to set his weapon to stun...but it was too late and now security from the mansion was after him.

He ended up killing 4 security guards and then found a place to hide out until the next session...

So no to forcing him to kill the kid, but maybe you flip the destiny point make him roll vigilance against a negative dice pool that you don't explain, if he fails you tell him a Droid asks what are you doing? If he succeeds you tell him it's an adult woman that asks what are you doing, and he only finds out it's a child AFTER he decides to or not to kill her that the being who caught him was a little girl, and that the sith spirit had used the force to mess with his perceptions. But the choice about whether to attack the being who catches him is still entirely his choice