You say this person, but you also use the word they, so it is one person or several people? Do you have other obsessive thoughts about other things? Is this new, or something you've gone through before? How old are you? Did something happen in your life at about the time this started -- something traumatic, something that bothered you a lot, taking a new medication, something in your family, something connected with this person or persons? This will help you understand if you have a chronic problem going on here or something new that might have a trigger somewhere. As for the thoughts, have you talked to anyone about it? Have you thought about talking to a counselor or therapist?
I think if you have thoughts in your head that you can not get rid of and are negative in this way, it is time to ask for help from a therapist. This is something to explore in a truthful way. For two reasons which include your being able to get past it and not have these thoughts anymore and also, in case the threat is real, a professional is aware and can try to stop it. Does that make sense?
There is something called 'avoidant personality' which is under the psychiatric disorders umbrella. Fantasies about hurting others falls under this as a symptom. It also can be a symptom of anxiety.
But either way, it is time to take action and reach out for help.
Anyway, I commend you for sharing this here. You are certainly not going to be the only one. We're here to talk about it in a safe way!