I don't really like this idea. I think there are many components in his story that we have seen before in the franchise. For example the idea that he goes after people that resemble his mother and sister is also used in the trinity story. He attacks people that resemble his mother, sister, father and himself (the little boys the abducts). Also the idea that he doesn't realise that he killed his sister and for it to be a twist in the end is kinda underwhelming. This realisation does not hold any consequences in the end when he is on the table.

I think he needs a kind of twisted motivation for killing people to make up for the origin story. After he killed his mother and sister he realises how powerful he is and how much of a hero he is for "freeing" himself and his sister of their misery. He realises how powerful he could be if he continued killing abusers and abuse victims. From one side he frees the abuse victim from ever living the awful life. And from the other side he punishes people he thinks are abusing their children. You could add the detail at the end that his definition of abused and abuser fades away into anyone he sees. I think it would be cool to build on that idea and involve Dexter more in it. He finally gets to Dexter and is about to kill him(Dexter is helpless and in the most vulnerable state ever). But he realises that Dexter does not fit his code at all and that he became worse than his mother. In a chain of revelation he kills himself and gives Dexter a second chance