I think the story was very disturbing to your young mind. It is a normal part of moral development to struggle with both the attraction of the power of a terrible deed and setting personal boundaries so we don’t do them. In your case, your efforts to not think about it makes you think about it. (The same thing would happen if I told you that under no circumstances should you think about a purple cow. You would struggle not to think about it, which would make you think about it.)
Since you haven’t been able to get away from the thoughts, it would indeed by helpful for you to talk to a therapist Trust me. Therapists aren’t easily shocked. We deal with difficult subjects and people’s pain about them every day. A cognitive-behavioral therapist can help you figure out what you are teaching yourself by holding onto the thoughts and can help you learn how to channel them to something more useful and much less stressful.
I wish you well.