The core book suggests you can gain corruption via evil deeds. Many of the scenarios (like those in Purge the Unclean) suggest the same. Do I have this right? I would think that it would be insanity that they're gaining versus corruption.

Corruption is used for things that negatively impact a character's 'soul'. Anything that brings them closer to the taint of the warp, like giving into the primal impulses represented by the 4 great powers of chaos will lead to corruption.

Insanity is a little different. You gain Insanity when presented with things that Should Not Be, or are pretty horrific. The fear table gives the examples of seeing someone walking around who you know to be dead and witnessing the scene of a particularly gruesome murder.

There's definitely some overlap - horrific daemons can generate Insanity when looked upon and also cause Corruption by their words or presence.

Purge the Unclean, IIRC, uses a mechanic that was later superseded. When DH was created and PtU was written, Corruption could include "evil deeds." However later it became only exposure to the warp.

To put it plainly the mishandled revision of the corruption mechanic as a way of specifically tracking chaos corruptioon rather than general moral deviancy is the element which effectivley makes Dark heresy into ordo malleus: the game, as apparently xenos, evil acts and blasphemy aren`t spiritual dangers unless there are deamons around.

I play corruption as a tracker of moral and physical degeneracy ergo if a character acts like a cold, compassionless, psychotic and morally bankrupt monsterI award corruption.

In extreme circumstances I even give corruption if players have seen or done something truly horrific but have avoided insanity with exceptional willpower rolls.

As a GM it`s upto me to choose how the rules apply in my group and that`s how it `s played out. Its up to you how you want to do corruption in yours.

To be fair, though, Corruption - as per the game's very rulebook - is supposed to represent "the corrupting taint of Chaos" as gained from "exposure to the warp, dark rituals, cursed artefacts and daemonic influence."

Certainly, there is an argument to be made about how "moral degenerancy" may act as a path to corruption in that it erodes one's spiritual purity - the only protection against temptation, regardless of its nature (and as such including the temptation of giving in to the allure of Chaos and the promises of powerful daemons).

However, it should be pointed out that "morale" and "evil" are very subjective terms that, in many cases, depend on perspective. If lack of compassion would truly lead to corruption, half the Imperium, foremost its most ardent defenders such as the Space Marines, the Adepta Sororitas or the Adeptus Arbites would be running around with tentacle arms, scaly skin or blood-red eyes simply due to their dedication - when actually, this conviction has the opposite effect, acting like a shield against corruption by providing loyal Imperial servants with a guiding light and a steadfast morale compass. It may be different from ours, but that doesn't mean it would have no effect on their soul.

Thus, if one really wanted to award Corruption for "evil acts", I would at least advise to look towards the individual character's personality, rather than the act they committed. Did they truly and honestly believe that what we as players might regard as evil was actually something which Imperial society teaches to be good, were they simply looking for an easier way to solve a problem, or were they even doing it because they enjoy inflicting unnecessary or even counterproductive harm just to sate their sadistic/egoistic tendencies?

It's a thin line to walk, and it can be difficult to discern good from evil when the way Corruption works depends so heavily on the individual's own opinion of their actions*, but in a way, this is what makes it such an interesting topic.

"One man's freedom fighter is another man's terrorist."

*: This reminds me of the Warcraft RPG and the Scarlet Crusade - there was a rule that the classes who tap into the divine force called Holy Light would immediately lose their powers if they ever "willingly commit an evil act". However, this Code of Conduct could be overridden in cases such as with the fanatical Scarlet Crusaders; in their zeal, they simply did not believe that what they did was evil, and so they kept their powers. The non-sentient source of their divine power did not follow a specific morale code, it simply reacted to positive dedication (the Greater Good, if you will), and as such could be "outwitted" by personal conviction. In essence, it was not the Light judging the character on whether they'd still be worthy of its gifts, it was the character subconsciously judging him- or herself, pitting their (potentially) critical and questioning nature against whatever web of lies and propaganda they allowed themselves to get sucked into.

And I believe the latter part is very similar to how spiritual corruption occurs in 40k. At least partially, for whilst simply "being an ass" should obviously never be sufficient to drop a mutation or two onto a character (and this is what Corruption as per the RAW ultimately leads to), it can potentially make a character more vulnerable to actual warp/daemonic exposure in that it simply erodes this "shield of faith" I mentioned above.

At the same time, fervent belief in a cause, and the justification of one's actions, may well eliminate any concerns one may have about a compassionless killing of supposed "enemies of the state/faith", even if they are unarmed civilians who simply had the bad luck of being at the wrong place at the wrong time. And without these concerns, however subconscious, there would be no erosion of spiritual purity.

Yeah, corruption just CAN'T be based on "evil deeds" as we would define them, or (as was pointed out) half the Imperium would be @ 100 corruption at any given time.

Yeah, corruption just CAN'T be based on "evil deeds" as we would define them, or (as was pointed out) half the Imperium would be @ 100 corruption at any given time.

This is a bad argument because it assumes the rules of the game are the same thing as the physics of the world the game is set in. Thinking this way leads to all kinds of silliness.

You can give corruption for evil deeds if you want, but be dang sure you're doing it consistently. A disagreement over why Evil Act A is worth 1 corruption vs Evil Act B being worth 5 and why Suspect Act C isn't worth any is a road you don't want to go down. Ultimately it comes down to whether you as a GM want to punish your players for doing evil things using corruption.

Also Lynata "morale" and "moral" are different words.