Is it bad to think of bad things (but not do them) ?

Is there a fundamental different between thinking and doing? (if yes, would that mean that someone who does something bad but thinks of something good while doing it, is exonerated?)

How can one take control of his own bad thoughts before they are to "be" in his mind, if not by thinking of those bad thought? (I wouldn't see "not thinking" as a solution but merely as a trick - if you don't think of anything like a buddhist monk tries to do, then you "cheat" when it comes to this question)