Where did I say he is purely evil? Of course he has some good in him, the series showed that. But why should I care whether a colonial master loves his daughter or not, his crimes far outweigh his good deeds.
If there is a truely "mixed" Cardassian character it has to be plain and simple Garak. He might have very well done more ugly things than Dukat in his past, but he is not as self-centered as him, brought the Romulans into the war and helped to liberate Cardassia.