The thing about Dukat, and the element that is at the root of what makes him evil (in my eyes), goes beyond the selfishness, the racism, the self righteousness, or the obsession with greatness. Its his critical unwillingness to examine himself. It doesn't just separate him from our heroes, it contrasts even eventually with his own kind.
Damar and Garak are two of the most patriotic and steadfastly passionate Cardassians in the history of the series, for all the good and bad that comes with that. Both extraordinary loyal to very flawed men (Dukat and Tain), enduring a lot for the nations, and loving their homelands more than anything. But both in the end still realized how fundamentally flawed the Cardassian Union was, and each one concluded that it was likely unjust for the old Cardassia to remain as it was.
That's not even going into those in the dissident movement or someone like Marritza who realized that "my country, right or wrong" cannot cover up the monstrous sins of an immoral entity. Cardassian culture emphasized a loyalty and duty to the state, but time and time again we see people look inside themselves, rethinking their views, and change whom they are.
Dukat never truly does this. He makes half hearted gestures with the sparing of Ziyal and the cooperation with the Federation/Bajor. But every major action of his always serves to reinforce his own pre-existing views. Even the shift in Waltz in the grand scheme of things is him simply doubling down on his internal beliefs: Not only was I not wrong for oppressing the Bajorans, I should have done it more. I didn't lose to them due the flaws in my actions/myself, it was the Bajoran Gods who defeated me. Benjamin Sisko isn't my better, he's the lesser Emissary to my status as the true Emissary.
Heck, the Cardassian he has most in common with is one he would have despised; Enabran Tain. Like Dukat, Tain is a steadfastly stubborn man whom never really critically examines himself either, spending his final days, blind and weak. He never admits the extent of his own mistakes, allowing only a small note of praise to his own son with his dying breath amidst the cursing of his enemies.
Any sort of redemption or genuine morality requires some introspection, which Dukat can't or won't do because he already decided his role a long time ago. Everything else is simply him determining what place the other players will have in his own grand drama. Kira and Sisko could have been his respective lover and friend, but chose to be his enemies. Ziyal was once his loyal daughter, yet has ended up a victim of his enemies in the Federation. The Pa-Wraiths were superstitious nonsense however they have become the guides to his destiny.
Trek is littered with evil characters that saw reflections of who they were, but still chose horrible actions. Dukat is a certain kind of evil that never even looked in the mirror.
