To start off, I'd say that in many ways he's better described as an anti-hero, or shall we say hero-villain?, because the character does have certain qualities - courage, assurance, intelligence, the force of his convictions - which in and of themselves are admirable. Many such people in real life do;
that's how they get others to follow them in the first place.
He has a lot of what it takes to be the great man he undoubtedly thinks himself to be, it's what he does with these in and of themselves admirable qualities that's despicable - and this is because he's also self-obsessed, power-hungry and entirely ruthless. He lacks a moral compass, and like most narcissists he lacks self-knowledge - the latter being the reason he ends up his own worst enemy more often than not.
Of course, from the first appearance it's obvious that this is going to be the main adversary, but the charisma the actor Marc Alaimo imparts to his role also gives the immediate impression this isn't going to be your run-of-the-mill villain.
Therefore the character could take development, but he didn't need turning on his head! For instance, "soft" I just don't buy from this person. The same goes for some sort of quasi-religious conversion. When he comes to Weyoun telling him he's no longer interested in power and conquest - I mean what's this man about if not power and conquest?! - that was it for me. I'd say that's where the writers lost me if they hadn't already done so by the end of the "Sacrifice of Angels" episode. I know the scene was meant to be moving, but I simply couldn't believe this man turning into a blubbing pathetic wreck from one second to the next.
I know the writers said it wasn't their intention to show him as losing his head over the love of his child but that's how it comes across. What made the character perfect for me was the fact that there were things that made him impressive while being reprehensible, so as a viewer you're forced to admire him almost against your will. It adds to his ambivalence: he may be a bastard, but he's a magnificent one. So he was someone you could really love to hate. There's no point in a good villain if you can't hate him and feel the precarious satisfaction of being justified in hating him. Yet paradoxically that needs a certain amount of respect. Strip the character of his dignity, and you can only despise and there's a difference. The fatal flaw in that scene to my mind is that he hasn't somehow fundamentally changed, he's not become likeable, you can't suddenly feel for him; therefore rather than making him sympathetic, he's made simply to appear pitiful. His apparent grief doesn't ennoble him or make him a better person. He is still vile; only now he's weak, and there's nothing more despicable than a creature who is both vile and weak. A spectacular fall from grace only makes sense if it can inspire compassion - all this inspired in me was a mixture of disbelief and disgust.
Ruining the character entirely just went in progressive stages from there.
First, turn him insane. Oh yes, the insane evildoer, lame cliche here we come. The truly frightening, and much more intriguing thing is the fact that the most atrocious deeds can be, and have been, committed and rationalized by perfectly sane people. Next, make him not merely insane, but the insane self-styled leader of some evil religious cult. I had my tolerance and my credulity stretched so far by that episode that I actually switched off the TV. Forget merely lame cliche, it's preposterous we're at now.
Next step, have the man who not so long ago professed his hatred of everything Bajoran in an outburst that, mental state notwithstanding, sounded as though he meant those words, willing to turn himself into one??
And to finish it all off, just go the whole hog and have the lame and preposterous and tasteless B-movie cliche of demonic possession, complete with the red devil eyes.
Why even invest originality and personality in the character in the first place if that's how he ends up? Why insult such a talented actor by wasting him on that kind of utter tosh?
What about the writer's much-vaunted avoidance of one-dimensionality? He's less than one-dimensional by the end, he's reduced to a caricature. Besides, insanity lets him off the hook, because you're hardly fully responsible for your actions if you're insane, even less so, if you're taken over by some malign spirit. It is lazy and counter-productive.
I want a character like that to be fully accountable and responsible for his actions and choices. Lastly, he doesn't even decently die, so there's no catharsis. The viewer is denied the one cliche he could justifiably expect, and the only one that would have been satisfying: the bit where the bad guy gets what he deserves. Only what he gets is the equivalent of eternal damnation - presumably his body burns up and dies but it's hinted that the essence of him is still in there somewhere with those fire things. Literally eternal torment? No-one deserves that. Even punishment for the worst crimes must end at some point because the crimes themselves cannot be endless. So his final fate is at one and the same time too much and not enough.
Simply watch the first meeting between Dukat and Sisko and the last confrontation and compare. That first scene packs ten times the impact and subtle meaning than the o.t.t. sledgehammer of the last. The first time they come face to face, volumes of antagonism are spoken with very few words, a look, a fleeting expression. In the final showdown by contrast, nothing very much at all is said with a lot of fireworks. Well, I rest my case...
Sorry for the length. It was a bit of a rant. I needed to get this off my chest for quite some while now!