I wouldn't worry about it. Look at all the incredibly grusome stories that are out there filled with horrible and disgusting acts. Those all came from someone's imagination. Horror writers are not out running around decapitating people in their spare time.
BUT... if you find that these thoughts are spilling over from your writing into your everyday thoughts then I'd say you have a problem. If you are concerned about yourself, book an appointment with a pshycologist and have an honest chat with them.