Evil thoughts are the product of your negative thought process. That produce anxiety. There are physical and psychological cause behind the onset of anxiety. Medicines can help in managing the physical symptoms of anxiety. But for those negative/evil thoughts to go away conpletely you need to undergo psychotherapy. Psychotherapy can help you to develop better insight to your problem and you can learn how to cope with those symptom. 

Evil thoughts can be anxious or obsessive or even delusions. Need more clarity. Treatment different for all