This is one of those stories where the context of being in Japan is everything.
Someone like this dude would have been hailed as a hero where I live, hell, he would've had the support of people even while in jail.
But in a country like Japan where the public image, social harmony, and anti-conflict culture is EVERYTHING, regardless of how "understandable" his action were, a label as heavy as "murderer" is not only gonna stick to him, but also to his loved ones (Like it was shown with his mother).
That's the kind of mark that will stay with his family for generations, twisting their public surroundings like a curse.
Add to this the mental distress that comes with killing another human being, on top of doing it in front of this monster's daughter (Which will have to live with the mark of being the daughter of a rapist) while also feeling somewhat responsible for what happened to his sister, and you have a whole recipe for disaster.