Absolutely true. There are particularly murderous Al Qaeda people who were operating in Iraq (and Afghanistan), and they are in an entirely different class (actually evil) vs. the majority of nationalist insurgents or common criminals.
Despite facing them as enemies, I wouldn't even consider most of the Taliban, Sunni former regime elements, or Shia militias to be evil for trying to kill the coalition military; the only really evil things they do are attacks on civilian population, and that isn't even done by the vast majority.
In Iraq, we actually successfully reached out to a lot of the less-irreconciable elements ("Sons of Iraq") and gave them a role in the new Iraqi state. There were cases where one day they were engaging US forces, and they next day were recognized by US forces as a useful security force and given equipment (well, reflective PT belts from the PX, so we knew they were the "good" guys now) and support.