Luther Harkon, maybe I just read over it, but didn't the NPC they murdered have a plot-function of sorts? If so, it's pretty easy to show your players that killing him maybe wasn't the smartest idea: their work gets more difficult. Maybe he could have told them something that better prepared them for running into this cultist lair or what have you, I'm sure you can think of something. You get my meaning, just make it abundantly clear that shooting first and asking questions later might not be the best way for inquisitorial Akolytes, in their own professional interest.

Since it obviously was a comprehensible in-character decision, I wouldn't opt for insanity and definitely not for corruption, but I think what you're planning with the investigator on their tracks is definitely appropriate. No consequences at all would, as Spacebaty pointed out, be simply boring.

And yes, real humans are a bunch of sadists covered by a thin sugar icing of civilisation. Still, that fact does not relieve you or your duty as GM to set the mood at your gaming table. Roleplaying is a work of fiction, and it follows essentially the same rules for drama as more linear means of storytelling. So, if you (and your group) want an emotional backdrop of "life is cheap and the world is bad and hell we dont give a ****", fine, let them roll like this. If you're aiming for something more... refined, for moral greys, guilt, action, reaction, consequences (well, the Shakespearean stuff basically), then no, don't let them get away with this.

Of course, just my personal 2 cents

Innocence proves nothing!

Well, yes, that is what he did, and I was a little surprised as well; though I consider this as a good immersion into the role (by the player) and less wanton destruction (i.e. the player is not a 13-year-old who was giggling while making this decision).

It is inhuman, but I suspect this was the point. The Tech-Priest is so inhuman, that he saw the Colonel as a nuisance similar to a virus/bug in a computer system that had to be eradicated.

Killing his servant while fleeing was a reaction by the rest of the party and those considered him the culprit. Actually the Assassin killed him with a head shot, while the others wanted to capture him alive for further interrogation (but they get used to the fact that the Assassin always kills and never leaves anyone for interrogation). He wanted to leave no possible “witnesses”.

Besides, it was clear the Colonel had no valuable information and that he was of no use and neither could help them in any way. He was very eccentric and aloof to the degree that he tended to not even listen to anything the PCs said. His servant even pretended to be dumb, because it didn’t matter anyway.

In the view of the Tech-Priest it was not crazy. Actually it was the opposite in his view. He comes from a strong caste-like background actually.

The cult plot will be “daemonic” enough, so that I want to have more mundane repercussions (i.e. the Commissar) for the Tech-Priest.

In that case I stand corrected. Good call and kudos to the player

Luther Harkon, maybe I just read over it, but didn't the NPC they murdered have a plot-function of sorts? If so, it's pretty easy to show your players that killing him maybe wasn't the smartest idea: their work gets more difficult. Maybe he could have told them something that better prepared them for running into this cultist lair or what have you, I'm sure you can think of something. You get my meaning, just make it abundantly clear that shooting first and asking questions later might not be the best way for inquisitorial Akolytes, in their own professional interest.

Since it obviously was a comprehensible in-character decision, I wouldn't opt for insanity and definitely not for corruption, but I think what you're planning with the investigator on their tracks is definitely appropriate. No consequences at all would, as Spacebaty pointed out, be simply boring.

And yes, real humans are a bunch of sadists covered by a thin sugar icing of civilisation. Still, that fact does not relieve you or your duty as GM to set the mood at your gaming table. Roleplaying is a work of fiction, and it follows essentially the same rules for drama as more linear means of storytelling. So, if you (and your group) want an emotional backdrop of "life is cheap and the world is bad and hell we dont give a ****", fine, let them roll like this. If you're aiming for something more... refined, for moral greys, guilt, action, reaction, consequences (well, the Shakespearean stuff basically), then no, don't let them get away with this.

Of course, just my personal 2 cents