It seems to me, as a prudential matter, that since Pope John Paul II said that conditions in advanced Western societies are such that the death penalty would not likely be necessary to serve the purposes of justice, an unsettled polity such as Iraq right now might in fact be a society where the death penalty may well be necessary, since if the government is overthrown, it is not clear if the perpetrator of crimes like this assassin would get justice under a change of regime. I believe that the death penalty for Saddam and his sons are justified for the same reason. Just my two cents.
I understand your political view of the situation; however, it does not follow that we should support it. If anything, one should be impressed by the fact that the government has acknowledged the error in killing another human over difference in religious belief. That is progress!