Nice to have so many people agreeing with me on this point...
To me the whole attraction of Dukat as a character was his ambivalence - the fact you could despise his actions while admiring him almost against your will.
Evidently, he was a bad guy - most people have a few
redeeming features, and the fact that he had those as well doesn't change the fact that he was essentially a ruthless, power-hungry, self-obsessed bastard.
And, of course, he
believed he was a good guy. That was what made him so realistic. Everyone thinks their own actions are justified.
And certainly your classic dictator types never think of themselves as evildoers. Hitler was convinced he was a good guy. So was Idi Amin. It goes with the territory.