Paraphrasing pancreasnowork, who's becoming my go to favorite lore tuber when it comes warhammer fantasy, nagash is not a villain who has some sort of tragic backstory, which led him down the path of villainy.
Nagash is an asshole, driven by greed, for absolute power and immortality. Plain and simple. And seeing as how this is the world of warhammer, that means, even after causing suffering on an unimaginably scale. After singlehandedly causing the destruction of his entire civilization and even running things for most during the end times, as well as after, he gets rewarded with godhood.
Because warhammer.
Having gone into the story, knowing full well what the end result was going to be, is felt almost sorrowful, seeing the warrior kings of Nehekhara go through hell and back, to defeat this human genital, only for it to be at best, a bittersweet victory.
And seeing as how in this world, nagash is the OG necromancer, even when he is defeated again and again, he has the nasty tendency to come back stronger.
At this point, I'm almost scared to pick up the following books and read about their tragic tale to be of Alcadizzar, the hero of Nehekhara and the witness to it's extinction.
But at least, hey, Settra does not serve. He rules, baby.