This is one of those things that I love/hate about 40k.
Great scary badguys, awesome...
But what do they do in their down time? Can you imagine the guy with shark teeth and no lips or nose relaxing and having a bagel with the plague marine whose gas mask fused into his face?
These guys are just all bad all the time. You can't get that armor off once your tentacles start putting down roots in there. What do they do when they're not on a battlefield being evil and menacing? They have to do something; they can't be fighting every minute of every day.
Also, they must all reek like absolute ass...like a porta-potty filled with tampons. All of them. nurgle doesn't need explanation, a khornite is going to reek like sweat and gore constantly, worse if you're fused to your armor and thus incapable of bathing, slaaneshis are going to reek of sweat and sex and blood and puke and shit and just about any bodily excretion some freakjob can make a fetish out of...which is all of them. With tzeench you got your alchemical reek of the infernal magical research and whatnot, plus of course epic BO, because they're not just nerds, they're ultra-mega nerds who believe they've surpassed humanity and thus don't need to conform to their standards...and of course whatever ichor and slime smells like because they'll have so damned many chaos spawned mutants running around. When your rectum morphs into a screaming garbage disposal made of shark teeth it can't be good on your ability to retain your shit.
No, I just could never picture myself being a chaos marine