I have cats, and I keep them inside. I love them, they’re great, but they aren’t for everyone. On top for that, they’re little serial killers that will kill just for fun and not bother eating what they killed. I’m sorry because I know losing Milo was super painful, but you should have kept him inside. Unless your neighbor broke into your home and poisoned him, that is actually on you. And seeking out “revenge” for a crime he didn’t do is something extra.