Something I have been kinda curious about for a while now. Now there probably going to be parties with members dedicated to different gods, and there are probably going to be PC groups that have person devouted to Khorn and a pysker (let's assume undivided for sake of argument). Here's my question, how does that work?

Without too much difficulty unless the Khornate is a real fanatic (in which case he'll be a problem in general). Remember that Khorne despises psykers, but that even devout followers of Khorne still ally with them (example: Horus Heresy and every single Black Crusade). And just because a character is aligned with Khorne doesn't mean he's devout. He could, for example, share some beliefs or almost none and instead be a violent, power hungry opportunist who is willing to feed the Blood God in exchange for power to accomplish his personal goals. If the character is sufficiently devout that he would look to whack a fellow PC psyker just for being a psyker, you're going to have other issues. Like constant murder for blood and skulls, whacking Slaaneshi allies, etcetera, messing up subtle plans with unsubtle bloodshed and so on. Now some conflict between players is fine, even if it does escalate to character slaughter, but if the Khornate player insists on murdering other PC psykers just for being psykers, then the problem is the player.

Every Black Crusade group is going to have issues of internal conflict. Khorne and Slaanesh, Nurgle and Tzeentch, tactics and strategies and so on. That should be fodder for good RPing, not an instant escalation to inter party violence. If the Khornate is suspicious of the tricky Tzeentchian sorcerer, that's appropriate. The Khornate reacting violently to perceived betrayal is also fine. Playing such a deranged fanatic that he'll try and murder and sorcerers and Slaaneshi characters out of hand is essentially forcing and extreme concept on the group and forcing everyone else to modify their characters in order to fit in with his and is, as they say, not cool.

Now now berserker, you won't get any Warp Time or Ecstatic Oblivion anymore if you keep trying to murder the little psyker!

Blood God's Contempt might pose a small problem, if the Khornate in question decides to take it.

Deinos said:

Today in Khorne News: Babaroth of the World Eaters, one of the top ranking candidates for this year's Khornate Daemonhood Award, got disqualified upon suspicion of using the services of psykers. Mister Babaroth made no official statement thus far.

I've been looking at the old 80s Realm of Chaos books... while Black Crusade is much more inspired by such books than, say, the 4th edition Chaos Space Marines codex, the general feel of Black Crusade is that the PCs will be much more level-headed, and generally less weird, than the characters depicted in Realms of Chaos.

In particular, Black Crusade generally assumes the Black Legion-style approach of chaotics being able to complement each other and work together. I, personally, could definitely conceive of a system similar to the 1st edition affair, where each chaos alignment is fully self contained and fully balanced to function alone. So for instance, Khornates might be required to kill or harass psykers, but would have their own ways of gaining large amounts of Unnatural Weapon Skill and Unnatural Strength. Likewise, the old paradigm of "gotta kill psykers (if Khornate), gotta kill opposing alignment types," also assumes that the gods are constantly interfering in your life, and constantly granting gifts (whether good or bad) after every battle, etc.

Deinos said:

That's pretty much it. There is no "Canon Law" of behaviour for a worshipper of Khorne or anyone else- there are things YOU do that will make the patron god happy if you did it, but as far as a game effect goes there isn't really anything that's going to rain down immediate divine fury if a Khorne worshipper works with a Tzeench sorcerer to "git er dun" when they go off on a mission and if the mission happens to be dedicated to Slaanesh, oh look, nothing bad happens to your character. Fail the mission because you antagonised each other's career, got punchy halfway through the job about who's god is best, sucked and got wiped by Imperials... well, those gods will let you know all about it with a big **** sandwich of Corruption!

Khorne actually has a lot of roleplaying aspects to it which dont have to be the meathead in armour with an axe either as there's dozens of ways to be a murdering bastard that makes people leak blood.

I prefer the 'Black Legion' approach. It is far better rp wise I feel. Adding variety and interesting interactions.
And I rather liked the latest CSM feel. The execution was questionable but unlike the 'bashers' I can see and appreciate some of the ideas in there.

Each and every player character is a cut above an average example of their kind - so if they are a khornate warrior they wouldn't be a mindless berserker, more likely a cunning general who will cause wars with untold bloodshed, or an assasin who can decapitate the leadership of even the mightiest foe thus enabling assaults on previously untouchable locales. The khorne berserker concept really should be below any player character and to be honest if all they care about is melee combat then they are better off going Slaneesh and being an insanely good duellist.