Why was A-bombing Japan necessary?

It was one of the worst crimes in the history of the world. Imagine yourself in their position? For example, many hate America today as they are the "world police" and are doing pretty much what Nazis' were doing but for a different cause (the oil and profit). Now, Imagine Iraq dropping two A-bombs on for example Washington and LA? And it would have been the same thing. You can't defend the cause. About 250.000 CIVILIANS were killed by a monstrous act of a madman (perhaps in an attempt to intimidate SSSR). Also during the Okinawa campaign many dead Japanese were desecrated and mutilated. For example by urinating on them, shooting corpses, or taking Japanese body parts as souvenirs or trophies. Japan was already defeated and dropping the bomb had no purpose. Was it really necessary?!?!