And who is to blame?
Most of the Sunni insurgents are former regime elements. Saddam didn't rule by himself you know. He had a massive security organization that gave him the power to rule Iraq. When you see the naked violence of the Sunni insurgents in plan view, remember this is how Saddam kept power. By these very people.
Most of them may have melted away during the invasion, but their blood thursty habits didn't. The same thing happened after the US Civil War. After the fighting was over, many thousands of men acustomed to blood letting went West. Thus, the "Wild West" was born.