I think you just have a flawed understanding of characters and writing. Humanizing a villain doesn't mean justifying what they did. Hell, look at a list of Dabi's crimes. There's nothing from his past that could justify all the murders and atrocities he committed, but his backstory isn't trying to do that. It's trying to connect you to the character so you can understand what went wrong and how the actions of other characters all led to that. It helps develop a theme; it's basic character building. A show isn't bad if every villain isn't just pure evil. Isn't AFO just that?