A corpse.

In a genestealer cult

I'd be an evil-minded sociopathic bastard with a deepseated distrust of others....so basically the same as I am now

Wonder if someone would be an Interax?

The Interex were completely annihilated by the Warmaster

Possibly a dark angels captain, a librarian or some kind of mutant.
I have alot of things I'd like to be, I could write a very non-cannon story about it.

There's a contest about that, y'know

Hmm so many ways to be ded.... but id either get sucked up into the Imp guards...or a damn pen pusher(then picks up that trusty Doom shotgun and fights off cultist that invade my pen pushing building of destiny!)

if I'm lucky as all hell? In a space marine chapter.
if I retain my current personality through this lucky transmogrification? Renegade space marine at best, traitor marine group/corpse most likely.

Most likely scenario: Corpse, or worse.

Probably living in the lower parts of a Hive sh*tting bricks while hiding from all the mutants and God Emperor knows what.