Was he killed in Iraq or the States? I think he should be killed either A) by the UN for war crimes, or B) in Iraq because he's an Iraqi and the crimes were commited in Iraq.
The US shouldn't have the power to excute a former-leader (I know... I know... but let's be loose with this term for a bit) of another country. However his home country Iraq, should definitely by able to give him JUSTICE.
Edit: Option A was assuming UN has death penlties regarding war crimes or crimes against humanity.
Saddam was tried in Iraq by an Iraqi court. The actual execution took place in the Green Zone, which is a heavily fortified American area in Baghdad.
You could argue, as Saddam did, that the Iraqi court and current Iraqi government is but a puppet regime of the US, but that is a matter of perspective and debate.