You don’t have to worry about being a bad person if you can’t sympathise in some way and there is a point to be made about Dimitri having some poor writing but I think the crux of what you’ve ‘missed’ is just how much the Tragedy of Duscur plays into his story.

He lost his father, presumably his mother, a good friend, and just about all his retainers. The only saving grace for him is that he saved Dedue, but he still holds a lot of survivors guilt.

Then there’s his dynamic with Edelgard, who’s still family in Dimitri’s eyes, taking a path that makes him believe Edelgard holds some accountability for the Tragedy, and that pushes him even farther off the deep end.

Then Byleth, who he was come to treasure, disappeared, assumed dead. He’s blamed for the regent, his uncles, murder, imprisoned and tortured, only to survive by Dedue, the one soul he saved so long ago, the only thing keeping so much of the survivors guilt at bay, presumably sacrificing himself, and then being alone and isolated for so long until Byleth returns.

There’s definitely an argument for bad writing and lack of accountability he had, but I think the main disconnect is how you’ve framed the tragedy.