Saddam wasn't hanged because of the people he killed during war. He was hanged because of human rights abuses including genocide. Specifically, he was convicted of the execution of hundreds of thousands of Shi'as that he had executed after the Gulf War.
As for Bush, the death toll is in part Bush's fault, but US soldiers aren't doing most of the killing. Most of the deaths are at the hands of other Iraqis. The two situations are entirely different.