Let's say you are engaged in an anarchic civil war. One of your enemies in that war offers you weapons if you promise to use them to fight another of your enemies. Do you take their weapons? Of course. Do you use those weapons to fight the enemy you were fighting anyhow? Of course. When that enemy is defeated, do you turn those weapons on the enemy who gave you those weapons in the first place? Don't be silly, of course you do.
Arming some Sunnis who hate the American occupation in Iraq to fight other Sunnis may seem like a sound strategy. It might even seem to work for a while. But, they are not going to hate America any less because we have given them guns. We have still bombed their villages, killed their sons and daughters, and stolen their country. The day will come when those American weapons will be turned on American soldiers. Don't be surprised.