You see, the rationale of that statement is that people in society will form vigilente groups, and what the vigilente does is NOT controlled. The criminal has far more to fear from torqued off members of society, than from the police. But, historically the vigilente groups have 'punished' the wrong person, and having the 'criminal' torn limb from limb is not uncommon. So, the police exist to uphold the law, and protect the criminal.
You're right. This will be Iraq's big challenge. If former criminals get to the justice system first, they might spare the nation a civil war. I have a feeling that there will be a lot of lynching parties and a lot of former henchmen that won't give up. The henchmen might not even give up to spare their own family, but some might if given the opportunity.