As others said before, he's an innocent person. I'd dare to say his heart is pure, but being tortured by the man he ended up falling in love with (which is, fundamentally, a pathological condition) then being forced to decide where his loyalty lies... isn't sad? He's a loyal person, but his own family is of a kind that deserves no loyalty. Still, to betray family is... a heavy, heavy burden. Yes, Igen felt in love with him, but only after he had destroyed most of him. They are gorgeous together (the art is amazing beyond words) but... the story itself is sad and nothing else. It's the story of a man who feels and what's worst, knows... he has no dignity, no worth beside being someone else's lover. And said lover is the murder of his own family. You might reply they did the same before he did. True, they all belong to the same kind. Kassan is the only decent human being in this story, beside his brother's unfortunate "girlfriend". They are two of a kind. This is a sad story form any point of view. Igen lost a lot, suffered a lot, but he's not innocent. Men like him are those who destroy the world, not those who save it. In a single word, they live for revenge. And revenge is a virtue only in manga. Revenge never becomes Justice. This is my personal opinion, of course, please take it as such.
Kassan's destiny is just.... too wrong. His soul is being ripped from him. It's just... wrong.