I think the only sensible answer to "Who has the worst fate?" is "Every character that ends up in Hell". An eternity of being tortured by Ciphrang seems pretty self-evidently worse than anything that any character in the series suffers while they're still alive.
Apparently there's degrees of damnation even. Some are more damned than others. So maybe Kosoter has the worst fate?
Saubon was a child pretty much till the moment of his death.
Well put. It's why I find Saubon a more sympathetic character than Proyas. Proyas is a man with every advantage and no excuses; he's smart, handsome, educated by Akka, loved by his parents and everybody else as a child, and winds up choosing evil because his refusal to doubt, to question, leads him to conclude evil is righteousness.
Saubon is an abused child who sought the trappings of strength and martial glory as an adult to hide the feelings of shame and inadequacy that abuse created.
I find them both sympathetic even if they're both terrible people. He doesn't just want to be superior mind you. He's not a narcissist. He wants to be good. Proyas' drive to be just and good was so desperate that it destroyed his capacity for reason entirely. But he never did feel secure in it, I think. Akka got through regardless, and so he pretended even harder to be the moral arbiter to compensate for his own fears.
I wonder how he would have developed if not for Kellhus.