Emilie I first want to say that the thoughts your having aren't normal. I'm a collector of knives as well. So there's nothing wrong with that.
But this talk of murder. And the fact that you're planning it out. That shows that you're not only thinking about it but you yourself mentally think it's ok to have such thoughts. The fact that you're enjoying killing them further lets me know that you need to talk to someone.
You keep letting something like this fester and you will act on it.
If you feel like you're going to be a danger to yourself or others you need to seriously consider seeking medical/psychological help.