No, its not ok to have bad thoughts, even if you don't act on them. With every bad actions, its thoughts that preceded it., unless one is insane, thus his or her action don't require much thought.
Take for example Timothy McVeigh.,or the Columbine Massacre., or lynching of the blacks during and after slavery., and yes the Holocoust of the European Jews , the genocide in Rwanda, the apartheid of S.Africa and so on. All of these events were premeditated, and are the results of built up anger/fantasy/hatred/bad thoughts....
Remember the saying, mind is a terrible thing to lose. The more you have bad thoughts, the closer you get on acting on them, Best thing to do is to remember and think of your creator.,and not go the path that Satan wants you to go. You can either do right and please god, for a guaranteed reward.,or fulfill satans dream of bringing as many human beings along with him to hell and do wrong.
Bad thoughts are evil, and therefore are not ok. Good thoughts are Godly, and easier to digest. God Bless!