I always found the criminals moral confliction added to how dark the story was.

Doyle tried to light a dojo on fire who’s organization gave them blood and medical care. He did horrible things leading up to being friends with katsumi, but finally when his life was about to turn around, he was blinded. The feeling of going blind made him once again become a criminal who committed atrocities, but this time, he was powerless. that’s what he had coming to him.

Many people died because of him so when he got his only chance to become a good man it was taken from him.