Is it normal to fantasize about threating/killing people?

I don't think it's a symptom of autism. It could just be that you have anger issues and make everything more dramatic. Unless you actually like the idea of killing, the words don't make you a killer; just an angry person. You can receive help for it if you like. If you do actually enjoy thinking about killing people, then that could definitely be a sign of being a murderer, so please get help for that.