I’m so very, very sorry that you lost your father at such a young age. I’m glad you wrote. It must be very confusing and frightening to find yourself thinking like a murderer.
It may be that what you are experiencing is a form of post-traumatic stress disorder. One possible symptom is intrusive re-experiencing of a traumatic event. Your mind may be trying to make sense of what happened to your father by identifying with the murderer.
Medication alone isn’t going to help. You need to work with a therapist to deal with the unresolved grief and horror of losing your father in such a violent way when you were young. Medicine may calm you down some so you can do your therapy but you will still need the “talking cure” as well.
The good news is that people can and do recover from PTSD. I very much hope you will research PTSD and then find a therapist to help you. You deserve to be free of these thoughts and to get on with your life.
I wish you well.