To love a yid: you need a rav. You need someone you can say these things to on a regular basis to clarify if you are correct or not.
If you check out the Artscroll on Av Harachamim you will read that real nekama is up to Hashem. We are not vengeful or out for blood. On the other hand, we must hate evil, we must protect ourselves and society, and do that by appropriate means. If part of the process will be helping this monster get the help he needs, so be it. But he will have to live the rest of his life under tight supervision. Someone who’s familiar with protocol for the criminally insane might be able to better comment.