system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] What would be the best way to fix this hellhole of a galaxy?

So Warhammer 40k is a universe where everything went wrong. It's the literal "worst timeline", and life sucks for everyone. If you were living in the 40kverse, and you could go back in time at any point in history and change one thing, one thing that would have the biggest positive impact on this universe, what would it be?
gpt
Kill Kor Phaeron before he kidnapped young Lorgar. This prevents Lorgar growing up to be a religious zealot and stops his legions love of worshipping anything that could be a god. This means no Erebus, no Erebus means no Horus being stabbed and almost dying and being corrupted by the forces of chaos. Already the Horus heresy is stopped meaning no empire destroying war. This also means Magnus doesn’t have to do anything like warn the emperor thus ripping a hole in the webway project. With no civil war and no daemons trying to get into the palace. The emperor can finish his plan for humanity dramatically reducing it reliance on the warp, whilst further advancing humanity to being a psychic race. 

Another way would simply be make the old ones save the necrons so they don’t become bitter and kick off the war in heaven which prevents the warp becoming so violent and stops races like the orks and Eldar being made. No orks always a good thing and no Eldar means they can’t murder fuck a god into creation.