People can be charming, even psychopaths
Especially psychopaths, it's what makes them so dangerous. I actually think it speaks volumes about Marc Alaimo's performance that so many felt this way about Dukat, no matter what he was shown doing people (both characters in show and the viewing audience) were willing to trust him.
The cave scene is classic narcissistic stuff, with him feeling the need to seek justification from his perceived nemesis. Nothing he is saying has any real bearing on his moral culpability, merely indicates he feels the need to justify his behaviour to himself and a select few whose opinion he deems worthy. In this case the man he perceives as his alter ego - the other ruler of terek nor.
There was a documentary made a while back where Hitler's secretary was interviewed at length about her time with him in the Berlin bunker and it was clear she was still in love with him, she barely mentions the atrocities she was asked to forward orders for, rather she talked at great length about how kind he was to kids and animals, how he was such charming company and listened to everything she had to say. Had he not been so charismatic it's difficult to see how he could have convinced half the world to follow him. The most dangerous people get to be that way precisely because they get people to let their guards down.
Yes Dukat may have tried softer methods, but only to assess their usefulness in gaining control and maybe reinforce some form of Messiah complex, not out of any genuine concern for the Bajoran people, you can guarantee that. Psychopaths don't always resort to violence as a default, rather they choose whatever methods they deem most likely to gain the results they desire at the time. Yes he may have been acting according to the policies of his government, but how is it that he, of all the billions of Cardassians out there, found himself in that singular position of holding ultimate power of life and death over so many? It's simple, because he was eminently suited to the job and had worked his way up callously disregarding life and suffering except where it suited him or his ego to do otherwise.
Tellingly, he can superficially respect and honour individual Bajorans (treating Kira as an equal when he needed her, bedding Winn Adami when he needed her, fathering and raising Tora - after trying to kill her) whilst subjugating the entire race, instituting mass murders, slavery and systematic rapes with equanimity. Again, he can be whoever and whatever he believes is needed to reach his goals, if that is charming friend and confidante so be it, if it is cold uncaring monster likewise.
The character to me constantly screamed out "redemption" and that is exactly why people loved him, they kept wanting that moment when something or someone showed him the light, his Darth Vader moment. But it never came because he truly was evil, merely very good at gaining people's trust.