I think I have a decapitation fetish.
That's peculiar! How is that different from your old BDSM fetish?
There is no bondage involved in decapitation.
Well, I appreciate that you're experimenting with your sexuality.  However, a fetish that involves causing fatal harm to someone else is probably not a fetish worth cultivating.  Try and find something a little less dangerous!
But I like the idea of doing it.
Yeah, but think realistically, if you harm somebody with your fetish, especially if the harm is fatal, you will have a hard time finding people that will want to do it. You will also face some serious consequences and you will not like them.