If these thoughts are things you'd never actually do, which means you don't truly have the desire to act them out, then I wouldn't be all that concerned about it. Sometimes I have bizarre thoughts that I think are horrible and even find myself thinking "Why on Earth did I think something like that!?!". Alot of times it's the odd desire for a shock reaction......full well knowing I'd NEVER do it and don't truly even want to. I think alot of it comes from the things we're exposed to on TV for shock value. I wouldn't spend much time worrying on it unless the thoughts become consuming, or you start actually considering acting them out.