There is no such thing as an evil thought. Or, maybe to say it another way - the evil in thought exists only in the condemnation of said thoughts.
As soon as a thought is classified as evil, there is resistance to that thought. That thought then begins to gain traction, substance, power. The more the thought is resisted the greater it becomes and the more likely it is that the thought will transform into some action.
Conversely, you can have all the horrible thoughts that you want, and if you allow them to exist, they vanish into the emptiness of their own insignificance.
There is a Buddhist quote that says something like,
Be careful what you say. Words have a way of turning into physical things.
Thoughts are basically words spoken to ones self. These words can be incredibly powerful and can (and do) devastate human life. Think about the evil actions that take place in the world. They all spring from either internal or external conversations. It is impossible to take an action without some accompanying dialog (internal or external).
Note: I am not including Zen-like states of action without an actor.
That said - looking at my above statement about words vanishing into the emptiness of their own insignificance might seem contradictory.
However the power of any dialog can only exist if the listener grants the words weight. If someone says to me, "You're an assh*le!", that may or may not impact me. The only way it will impact me is if I repeat it to myself and give it substance.
I can either give it substance by resisting it - I'm not an assh*le. What do you mean? How can you call me an assh*le when you are always..blah blah blah - or by succumbing to it - I guess maybe I really am an assh*le. God, what a loser I am. Maybe I shouldn't even try...blah blah blah. In either case, it is the power of my internal conversation that grants any power to the external conversation.
My thoughts are extremely powerful in the above case - and they unquestionably have an evil edge to them. Either they are weighted with self-deprecation or self-protection - both of which are a mask or buffer between life and the SELF (God would perhaps be an appropriate synonym here).
But if instead of any of the above nonsense, I relate to the external conversation as it is, and give it space to exist without identifying with it, then it will disappear and it will be meaningless - something like Oh, he said I'm an assh*le. Then the thought/words are not evil and contain no power.
There is no requirement that another person be involved for this type of scenario to manifest. I make a mistake - I have a thought; I see something that reminds me of a personal failure or loss - I have a thought; I am sitting in my car with breeze blowing in my hair - I have a thought.
My father suffers from OCD. He will have images in his head of murdering his wife and son (he remarried and has a young child - my brother). These are terribly painful images that he will never act on, but that have nonetheless caused him a great deal of anxiety. He was terribly ashamed of having thoughts that seemed to come from out of nowhere, and he thought the fact that he would even have such a thought meant something terrible about him.
Then one day he had an insight. He realized that thoughts don't mean anything. Since he knew he would never do what these mental projections suggested, he began to relate to them the same way he relates to a sneeze or a burp.
All that happens is that something floats across his field of consciousness. This something has no weight nor any bearing on anything real. When he has an "evil thought" nowadays (note: their frequency diminished greatly once he stopped resisting them), he just says to himself, "Thank you for sharing", and lets the thought pass; and it does - soundlessly vanishing into the ether from whence it came.
The evil in a thought is never inherent in the quality or content of the thought - but in the response one has to the thought.
Something James Carse said seems apt:
"All evil is an attempt to eliminate evil".