Also Imperium directly fuels chaos on every level, so lesser evilism is piss poor explanation in this case. Old lore directly blamed chaos rising on humanity as the most chaotic species, and even looking at current setting it’s still true (just not said) because 99% of people and culture of chaos have human origins.

And just chaos was not to make Imperium look good, so Tyranids arrived, purest devouring swarm trope possible (predictable radlib reaction - stan Genestealer Cults, still will eat and kill everything, but with revolutionary aesthetics please ignore literal cult and brainwashing). Plus Orks despite all shenanigans are nothing but organic von Neumann machines. Plus Necrons, thankfully somewhat retocnned to not be just zombie apocalypse crossed with terminator but still very close to it.