I understand the argument that maybe the United States didn't have the right to kill Saddam. But let's not forget that he and his sons tortured generations of Iraqis. If Saddam were still alive, I'm sure he or his family would have made a gran for power somehow and the Iraqi people would suffer again.
Saddam Hussein was a dictator who used his power against his opponents in Iraq. He was a terrorist and deserved to die. The United States could have killed him when the invasion took place but instead their option was to hand to give him to the authorities to follow a trial and his final death.
The United States had the right to want Saddam Hussein dead. However, America did not actually kill the former Iraqi dictator. Instead, Hussein was put on trial in Iraq, convicted and sentenced to death by hanging. It was the Iraqi government which ended up killing Saddam Hussein, not the United States.
For a government killing person one might see it to be alright. But as like death penalty, none should be allowed to decide for someone else's life. Not even as a warning, not eve as "an example what happens if you don't obey". That is why I see all killing as a bad and a wrong thing to do.
The USA, did not kill Saddam. The new government tried, convicted and executed him. Because US did not offer Saddam asylum, nor the option of seeking refuge in a country like France—for example—you could make the case that US had a part in it, because US turned him over.
Whenever a new government is installed, following a war, or during a war, there is a purge by blood, banishment, asylum or execution. Prison is also available if it is deemed that the persons are not a threat to the government coming into power.
The change of power is really little different than the way Saddam came into power; a bit more democratic, perhaps, but not much different.