Intrusive thoughts such as this are common, everyone has them. The reason is because our brains are constantly having thoughts and we just don’t pay attention to most of them. In any situation your brain is analyzing all kinds of possibilities, and automatically ignoring the irrelevant ones for the most part. Occasionally you notice them and if they are scary or unpleasant it might seem upsetting for a moment but most of them time you can easily dismiss them. Unfortunately sometimes some people, usually due to mental illness, being in a temporary bad mental state, or for other reasons pay more attention to these kind of thoughts than they need to which can lead to anxiety, depression, and in unfortunate cases dangerous action. If you find yourself experiencing these kind of thoughts on a frequent enough basis (or they feel particularly strong) it’s a good idea to talk to a mental health professional. There is help available and you absolutely can do something about it.