You may not feel bad about it, but to answer the original question, the character who ultimately is going to suffer the worst fate of any in the series is Kellhus. Irresective of anything any character has suffered while alive, almost by definition, the character with the worst fate is going to be the one most damned, and as Sharmat pointed out, there are degrees of damnation.

The two factors that seem to contribute most to a character's damnation are the extent of their misdeads and the extent to which they have practiced sorcery. Kellhus is the genocidal murderer of countless thousands of people, perhaps more than any other human in the history of Eärwa. He is also the most powerful sorcerer the world has ever seen. Following the moment that his living body is turned into pillar of salt, I would suspect that his soul is set to be promptly and eternally subjected to the worst horrors hell has to offer from the countless Ciphrang eager for revenge on the man who would seek to conquer them and their domain.