i can relate - i have very violent fantasies when i feel angry. i seem to go through periods when i am intensely angry and have these fantasies, and then periods when i rarely get angry, and don't have them. so i think it might help to check how angry you are feeling generally, or if you have old anger about anything in your past, and maybe see how that might be coming out in violent thoughts towards people now.

i also have not told my therapist or dr the extent of my thoughts, for the same reason. one time when i was in an inpatient setting, but not a psych ward, i told someone about a fantasy i had in more detail, and was immediately made to leave because of being a danger to the other patients. so - although the sensible advice is always to talk about these things to someone you trust, the mental health field does not provide a healthy atmosphere or space in which to do this when it comes to thoughts like these.