Was Dukat really as irredeemably nasty as Sisko ultimately decided?
No, he wasn't. But unfortunately, the writers became hellbent on forcing the message that he was down the throats of all DS9 viewers.
That is why
they assassinated the integrity of Dukat's character beginning with Waltz, and continued to do so throughout the rest of the series.
As a result of that very, very bad decision, the Dukat character and a significant part of DS9 as a series became ludicrously fubared.