In Black Crusade, Failings give you Corruption points (which can sometimes be beneficial). Failing your Chaos God gives 1 Corruption point.

You fail Khorne by losing a one on one fight, NOT by being too honorable/dishonorable, too compassionate, refusing to face an enemy, succumbing to fear, etc.

You fail Tzeentch by losing a fight with psyker foes, NOT by refusing to lie, being unambitious, too obvious, refusing to manipulate to your advantage, etc.
You fail Nurgle by losing consciousness from fatigue or wounds, NOT by not being paternal, not spreading disease or even curing disease, etc.

But you fail Slaanesh by not indulging your desires and by not desecrating/corrupting enemies.

This is interesting because Slaanesh followers are the only ones who are required to behave in a certain way... one would think that all the chaos gods are alike with Slaanesh (so a Khornate would have a Failing not by being defeated, because Khorne doesn't mind if his own followers bleed for him, but they might have a Failing for sparing a psyker or some such), or that Slaanesh would

Here's what I wonder: is Slaanesh more demanding and different in behavior because he's an alien god who was born in the massive destruction of his parent species, while the other chaos gods are more temperate since they were created without directly physically harming humanity?

They all seem like codes of conduct to me. Don't lose, don't lose, and don't wimp out.

Well the others are punished for failing combat but Slaanesh is probably the least combat oriented of the gods (IMO) so he punishes for failing other things.

If I were DMing, I'd give someone a failing for any Roleplaying that's contradictory to their God, period. So in other words, I'd invent a code of conduct for all the Gods and punish the PC's for not following them (while of course rewarding them for following them well). I'd use the conditions already in the book but also tack on those things you mentioned above, and any others I find necessary. Make sure the PC's have a good understanding of their patron Gods however, or else they might feel cheated.

That's something I've always wanted to be able justify doing within a rule set, and it looks like with a little tweaking, you can in BC!

What exactly would be a Code of Conduct for the other gods, I mean besides "Hating Pyskers" if you worship Khrone and having four major schemes running at all times if you follow Tzeentch. I mean outside of Combat I imagine your average Khrone cultist isn't like a world eater (or even inside of combat if you look at the blood pact) .

Actually, I JUUUUUST noticed that the Mark of Nurgle and Mark of Khorne "drive" you to spread Nurgle's Rot or to shed blood, respectively. Though amusingly, unless you're a reasonably powerful/corrupted psyker you will have no way of actually spreading Nurgle's Rot.

PCs can avoid getting marked by Gods without becoming Chaos Spawn, right?

What do you mean? Such as Undivided chars?

Deinos said:

Yeah, also could you be a Khrone or Nurgle player and not have to get marked, or if you get marked can you resist the urge to Kill or Infect everything without corrupting so fast you turn into a Chaos Spawn. Mainly because I am worried that when I get my character high enough I would be forced into a very narrow line of conduct and if I go too much off it I get horrible amounts of corruption and or heavy brass things thrown at me.

Well, once you have 20 aligned purchases of a given god and 5 more aligned purchases of that god than anyone else, you get a Mark. The Tzeentchian and Slaaneshi ones seem kinda mild (the former are driven to gain knowledge, the latter are driven to persuade people to become motivated by lust and greed), but the marks of Khorne and Nurgle are probably more severe. Such characters are probably going to be safer if someone else is calling the shots... although PCs already are placed into the position of having to kill all the time, so I suppose its not too big of a hindrance.