Wow, I'm really sorry to hear this. It must be very upsetting to you to have these thoughts on a regular basis. Two things come to mind initially. First, these are intrusive thoughts. Meaning you don't want to have them, they just show up, right? And if you try to push them out or change the image/dialogue in your brain, you can't? Second, habits. I have an anxious son and he has thought process habits that he has to try to break (according to his therapist). Could you change this thought process habit? Could you change what you do at night before bed to try to change the situation?
Really, you do need to talk to a trusted adult in your life. If you have general anxiety, there can often be a doom and gloom feeling for no reason but it is frequently there. Most people have some bit of anxiety over some things but most people do not have daily bouts of picturing their death by knife! You should tell your mom, dad, both, an aunt, uncle, older sibling, etc. Someone. they help guide you for your next step.
Now, I have done this thing if I must be honest here where I picture something awful. Like one of my kids getting cancer. I mean, really. My kids are healthy and that is not anywhere close to reality. But if I picture it, I can feel the emotion of it and make myself cry. Everyone once in a while I have done this. And then I catch myself crying and am like "really?". Why?? lol And then I'm like "snap out of it" and move on. Can you do that? Can you snap out of this and move on?
The fact you mention the burglary suggests maybe it had more of an effect on you than you know. I don't know this, but you did choose to mention it. I'm guessing that, since you say you're not a chronic anxiety sufferer, something happened somewhere to make you think about this. Young people have really vivid imaginations, and who knows what you heard and took literally that wasn't meant to be literal, or what you saw or heard about? That's why it's not a bad idea to talk it out with someone, sometimes that's all it takes to de-mystify the thought.
Post traumatic stress syndrome is a thought as well. Talk to your parents. See about getting some therapy to discuss and go from there.