Recently, I've come to the realization that there is no such thing as a bad thought. There is no such thing as a good thought either, for that matter. Thoughts are simply, neutral. Sure, you can think of something that makes you feel good/bad, but I'm referring more to inherent good/evil.
Your thoughts don't have the power to help or harm someone the same way your actions do. There's a huge difference between "bad thoughts" and "evil deeds." You can argue that thinking is an action and since the word "think" is a verb, that's technically correct. But it would be hard to argue that thinking about giving that homeless guy some change does him just as much good as actually dropping a few pennies in his cup.
It seems the idea of thoughts being good/bad is biblical. I'm pretty sure somewhere in the bible it says thinking about sinning is just as bad as committing that sin, which is dumb. We all know that thinking about something and actually doing it have vastly different results. Planning out a murder in your head will never hold as much weight as actually going through with the murder.
Another reason I don't think there is such a thing as good/bad thoughts is how the human consciousness works. You can't fully control your thoughts and choosing to be good/evil is a conscious decision. Too many many of our day to day thoughts are passive. We aren't consciously deciding on the exact thoughts we have throughout the day.
In order for thoughts to be good/evil they would at the very least have to be fully conscious. Compare passive thoughts to a conscious action. If you know society's definitions of right and wrong, then performing a good/bad deed is a conscious decision.
At some point, guilt and come into the picture. Some people feel guilty for their "bad" thoughts, because they know of a person or group that would be offended by those thoughts. What people must realize, is no one is offended by thoughts they aren't aware of. Just because you think a "bad" thought, that doesn't mean you are a bad person.
At the end of the day, your thoughts don't affect the world outside of your mind. Shit, if you can't be completely free of societal bindings in your own head, then where can you expect any type of freedom? It's your mind; do what you want with it.