Well I think the thing here is that Sisko believes he is pure evil. So, whether or not you agree with Sisko's label, through his moral lense that's what Dukat is. I agree if I were in that situation, I might show a little sympathy towards Dukat, since he is obviously bonkers. Or I like to think I would. But remember, Sisko has been dealing with him for years, and obviously has developed a strong distaste for the man. He's injured, thinks Dukat is scheming to kill him or keep him there as a prisoner indefinitely. He clearly has never trusted him, and for good reason, Dukat is extremely slippery. As the emissary, he also will take Dukat's role in the occupation more personally than your run of the mill Starfleet Captain. I guess my point of all this is I can see why Sisko didn't act like a saint and try to help him with the obvious episode he was having. He just despises Dukat, and think he isn't worthy of redemption.