Question: Is it normal to have the thought of killing people in graphic ways?
Answer: That's normally called an intrusive thought; a thought you don't want to think about, but it forces it's way into your mind. So, yes, it's incredibly normal. Humans are naturally curious creatures and we explore a lot of concepts and ideas. Sometimes these thoughts are fueled by trauma, stress, or the intensity of a strong emotion. They are also signs of some disorders and mental illnesses, although you shouldn't immediately assume it is. As long as you don't act upon these intrusive thoughts, then it's perfectly okay to think like this. The day you consider acting upon or start thinking about them religiously is the day you need to ask a guardian or someone you trust for help.
Uplifting Note: We're all human, so next time you feel alone in your battles, know that someone else is struggling with the same thing! We're together in this.