Iraq has a democratically elected government. Whether or not they are accountable, rights-respecting, impartial or even just is up for debate. But they can't govern without being able to impose the rule of law through a monopoly on force.

Warlords do serve a social purpose. But it's better for everyone if a stable government serves that purpose instead. The nature of being a warlord is such that they won't respond to reasoned arguments or give up the power they've risked their lives to build. The only way to get them to step aside to allow democratic governance is to kill them or convince them to surrender.

Killing a warlord has nothing to do with the concept of justice. The term "outlaw" originally applied to people who lived outside of the protection of the legal system, meaning that they could be robbed or killed by anyone. If a local leader wants the protection of justice, they should run for office or go through an official process to wield the sanctioned power of the state. Otherwise, they should be prepared to either get down or lay down when just democratic rule comes to their region.