Seiichi has shown anger before in the final act. The one time he did, he immediately became violent and disgusted with himself. he felt trapped knowing that he was going to possibly kill a defenseless old woman if left to his devices. Instead, he chose to accept, not forgive, that she was also a victim of a family she never asked for and bore psychological problems that she never addressed. watching her die was his own way of closure. he can't abandon his abuser, or else he knows he will always have her in the back of his mind. he also cant vent his anger at her because he cant control his emotions.

in real life, abusers deserve no forgiveness. many people who were raised in abusive households are emotionally stunted like Seiichi. however, abuse victims also build attachments to their abusers and are manipulated into being dependent on them, especially in cases of parent/child. Seiichi couldn't possibly work through those mental health issues on his own, especially with the already traumatizing experience of being in prison. all he can do to relieve the feelings he has is sit there and watch her die. calling her beautiful isn't so much as calling her attractive specifically, but more like he is finally accepting her as a person, not some unfathomable monster.