Further to the below, the Wikipedia article on death squads gives a good picture of the sorts of monsters who use these tactics. It's basically a rundown of the twentieth century's worst genocidal maniacs: Stalin, Hitler, Pol Pot, the Rwandans, Milosovic, and of course Saddam Hussein, who used his Fedayeen Saddam to murder rebellious Shiites, Kurds and Marsh Arabs.
Such tactics are sadly not new to the US. In Vietnam, there was a little thing called the Phoenix Program, a campaign of indiscriminate murder aimed at qwelling the Viet Cong insurgency and eliminating possible leaders and supporters. And in Latin America, the US armed and trained death squads in Honduras, Guatamala and El Salvador, resulting in thousands of murders of leftists, trade unionists, and religious leaders.
If the US uses such tactics on the Sunni, Iraq really will have come full circle. Rather than ending Saddam's reign of torture and terror, the US will have simply installed an equally monstrous replacement, who will carry on exactly where Saddam left off. Plus ca change, plus c’est la meme chose.
I really do wonder what Sock Thief and his "decent left" friends will think of this - whether they'll somehow manage to doublethink it away, or whether they'll finally see where their "humanitarian" intervention has taken them.