>>718939805
Because that's literally the Imperium, as well as the DAOT Humans before them.
Humanity in 40k is not good, in fact it's not even properly comparable to modern Humanity given the several millenia gap in culture. The only thing that keeps the Imperium propped up is rabid and horrendous expansionism seeking to claim, rapidly harvest, and discard every planet that humans can set foot on. It's a bloated, festering, chambling corpse feasting on whatever scraps it can come across, and has been as such since the Great Crusade.
And at the head of it all is the Emperor, who the Imperium prays to for strength to pillage, plunder, and destroy everything in their way.