Traitor/Heretic

The most common Antag Role, a servant of KAYOSS or maybe just a Renegade.

What is I do pls Im scared first tator round
DON'T PANIC, NEWHERETIC. DAPPY IS HERE TO LEAD THE WAY.

As a Heretic, you, by default, worship one or all of the CHAOS GODS. How you go about your business is up to you, but usually, if you want the Favor of your god/gods, you can go about and do things as such:

So you want to be a Nurglite?
Nurglites are worshipers of Nurgle, god of Death, Decay, and Pestilence. Usually, Nurglites are friendly chaps who like to spread STDs all over the place like the Plague, and aren't generally mean or bad guys. Sure, once they're found out, they'll kick some Loyalist, but until then, they're pretty chummy. Nurglites truly shine in Virology, where you can spread disease and plague in the name of the Plaguefather, but if you don't have that kind of access, killing choice people (your Assasinate objective) and leaving their decaying corpse lying about is usually a good substitute. Nurgle's blessings are many, in the form of diseases, poisoned blades, and Nurgle Daemons.

So you want to be a Tzeentchian?
Tzeentchians are usually the least-played Heretic, since most people tend to love slaughter and bloodshed. Tzeentchian Heretics worship Tzeentch, God of Fortune, Schemes, Fate, and Sorcery. You usually are a very smart cookie if you worship Tzeentch, as you plan out your actions with endless intelligence and cunning, working out all the methods. A carefully crafted plan earns you Favor with Tzeentch, especially if it succeeds. He may even grant you a few Warp Shadows to boss around, some lovely mutations, a Bedlam Staff, or maybe even wicked, sorcerous powers!

RAAARGH BLOOD FOR THE BLOOD GOD
So you're gonna be a Khornate like everyone else. Seems legit.

Khornates are usually heretics who love the thrill of battle and bloodshed, carving a bloody swathe through their enemies in the name of the Blood God, Khorne. Being the God of Blood, Skulls, Honor, and War, Khorne loves those who fight and kill in his name. Now, there's a clause to that one. If you kill an innocent, one who can't defend themselves in righteous battle, Khorne will be very, very displeased. Other than that, glorious bloodshed is tribute to Khorne. Assassinate objectives should be carried out in a grand duel. Favors from Khorne usually are slashy melee weapons, armor, and HULK.

So You Want to be a Sick Fuck?
Slaaneshi Heretics are mostly known as nothing but harlots and rapists, but that's a Ban. The proper way to be Slaaneshi without a ban is to prey on Excess. Eat a lot, torture someone slowwwwly and painfully, do ALL THE DRUGS, maybe even force someone else to (your target, anyone else, the Clown), or make TONS OF NOISE THINGS SHALL GET LOUD NOW WE ARE AMPLIFIED. Favors from Slaanesh tend to vary, mostly in mutations, but items such as lashes, whips, flails, or torture equipment show up.

Fuck all that, CHAOS UNDIVIDED!
You do have the option of worshipping all the gods as a pantheon, in which you can do all of the above.

Fuck THAT, I'd rather just be a Renegade.
If you don't want to get heretical, there's always the Renegade option, in which you just traitor normally because you hate the Imperium.