Posting to watch, I'd like to see how people can justify actually caring about this universe where there seems to be no hope, everything is absolutely shitty and all the factions are almost equally evil.
Think of the average Imperial Guardsmen. He's been drafted from his planet into a giant cathedral in space, given some cardboard and a flashlight, and dropped on an alien planet, to fight magical super soldiers, giant bugs, or sick twisted murder-fuckers. The question posed is "Why does he stand and fight? Why not run, or cower, or join the enemy, or end his own life?"
Think of the average American in WW2. Why would he join the fight against the Nazi menace? Not just to serve his country, and to enact his duty, but to protect his family, to give them a life free from fascism. Take that logic, and scale it up to a galactic level. A guardsman isn't just standing between an Ork and his family; he's standing between that Ork and his entire species. Each second of humanity's continued existence has a carefully calculated cost, paid in the lives of Guardsmen.
The average Guardsman has only a glimmer of hope, and that can be enough. They might not miss, they might wound, they might dodge that explosion, they might make it through, they might come out on top, and give the trillions or quadrillions of other humans another moment of life. Don't forget, the only reason the Horus Heresy didn't succeed was because a single guardsman stood up to a demigod of war and hate, and took a blow for his Emperor, his God, but not before landing a single blow that enabled the Emperor to kill his favorite son. Each Guardsman who falls is Ollanius Pius on Horus's ship during those climactic moments, dying so that his Empire and Emperor could have but another moment to make things right.