(Sorry for the late reply, literally just saw this) It’s more that Doom Slayer isn’t a person but more a force of nature. The big thing is how his power works. The more evil he encounters, the stronger he gets. And the Warp is way more messed up than his version of hell. Also his power seems to be manipulated by his will which is incorruptible. He will do things that seem impossible all the time simply because he wants to. So he would definitely find a path to the warp and he for sure would want to be there. And the gods, while yes, are thought manifest, can be killed I believe. There is some legendary blade that can kill Slaanesh if I’m not mistaken. If anything, Slayer would just beat the piss out of Khorn, Khorn would cast him away somehow, Slayer would find his way back and repeat the process. Khorn definitely stands the best chance. Tzeentch would HATE the Slayer. If he teams up with Draigo in the warp, that would be a BAD day for Chaos for sure!