When I read your letter, my first thought was “What happened to this person that created this problem?” Generally, people who want to hurt others have been terribly hurt themselves. If you were seeing me for therapy, I’d want to explore whether your thoughts of violence are a way you try to protect yourself. Thoughts of violence certainly give you the feeling of being in control or having the upper hand.
If that isn’t part of the picture (or if it’s only a part of it), I’d want to talk with you more about the “switch” — tracking when and how it happens.
Obviously I can’t do any of this. I do think what you are describing is serious enough that you should make an appointment with a therapist who can. You don’t like these feelings. You are concerned about them. You are scaring people you care about. Those are all good reasons to get an assessment from a mental health professional. It may be that you just need some reassurance. Or it may be that you need some healing. You and the therapist will decide the best way to help you.
I wish you well.