All indications suggest that the USA wanted him to to be tried, convicted and ultimately hanged, for the even larger "crimes against humanity" he committed against the Kurds.
It seems a bit odd "they, meaning the Shiites, settled so quickly for his death for a mere 148 individuals, when the primary trial was ongoing for tens of thousands of Kurds.
I don't pretend to understand their "reasoning", but in all this talk of Sunni vs Shiite, little seems to be said of the Kurds.
I wonder why?
I have no love lost for any of the Muslim sects, but the Kurds must be seated prominently at the first table in Iraq.
IMHO, they deserved first dibs on his "execution".