George W. Bush was also an evil leader. Now, imagine if he had been shot and Arab people were singing and dancing in the streets.
Guess what? That's probably how the average Arab person watching this on television feels today. Yes, they know Osama was evil, but they know a bloodthirsty crowd when they see one.
The world is much better off without Osama, but someone getting shot in the head and dumped at sea is not an excuse to party.
Not only is it unseemly, it also increases the chance of American citizens getting blown up or shot in the near future.