Personally, he got the justice that most if not all the people expected, but never thought they would see. So regarding his execution. That itself was not the problem, but i think the whole climax and political circumstances surrounding it.
Here you have an iraqi sunni president, who opressed many shia. now his exceutioners, are shia, opressed and full of revenge. he is recitating a prayer, but half way through his executioners taunt him and call the name of sadrs uncle who was supposedly executed under saddam. They then let him hang.
Its just comes across as one side taking revenge on the other. Not the execution, because that was justice, but the way it was all done, the timing, the taunting and so forth.