Yeah sorry. Being crazy doesn't give you a pass on being evil. The crazy guy that thinks the birds talk to him usually isn't hurting anyone. Good and evil are based on what you do.
And what he did exactly is never resolved, all we have are his insane rantings. Sisko hasn't learned anything new except for everything Gul Crazypants admitted too after being led by the nose in that direction by Sisko. So what makes Dukat 'pure evil' now, and not before the episode?
Come on, you watched DS9... what did he do? Despite all his rantings about being "unappreciated" and "gentler" he was the tyrant who oversaw the occupation of an entire world, that employed slave labor working people to death, and routinely retaliated against the civilian populace when the resistance had the nerve to fight back.
He basically destroyed a child's life(Rugal) just to discredit a political enemy, not to mention his adoptive and biological families.
He was willing to execute his daughter just to maintain his position. And don't tell me he spared her for noble reasons, Kira had to threaten him to do it and even then I think it just fed his need to be admired by her and Ziyal.
Not to mention the whole... negotiating Cardassia's entry into the Dominion and attempting to launch an attack that would destroy all of Bajor and his daughter who he disowned because she "betrayed" him.
Oh yeah, then starting the largest conventional war in Trek's history after that failed in textbook revanchist style.
If you only want to go after his "breakdown" okay... plenty of material there too. Letting a pagh-wraith occupy your body to destroy the Prophets and incidentally murder Jadzia qualifies. Also making a call to mock Kira about sleeping with her mom was deliberately cruel and malicious.
Then starts a whole religion with is rooted on wanting to be admired and loved by Bajorans. Oh and tries to get them all to commit suicide when they start doubting him because he sleeps with one of his married followers.
Oh and then there was that little plan to team up with Kai Winn to release the pagh-wraiths who would then kill everyone they don't like.
You can argue if he's crazy or not, it doesn't matter. He was evil. Being crazy doesn't excuse that.