Who are the Iraqi terrorists, and one other question

So some Iraqis are choosing to be suicide bombers, while others are burying explosives under roads, and then detonating them when people they consider enemies drive by. Maybe I'm reading the wrong articles, but I wish someone would tell me if the people doing these things are all Sunnis, Saddam's no-longer-in-power Islamic affiliation. Or are Sunnis and Shias both taking part in these activities?

I guess the answer to this question would tell me whether the motivation for the insurgents is just to get the invaders out of their country, or are the Sunnis also attempting to strike back at the group that replaced them as the ruling group in Iraq (the Shia).

My other question involves the concept that if America withdrew from Iraq, the situation would worsen, and likely lead to civil war. Or is the fear that Iraq's feeble form of democracy would be snuffed out by some Saddam wannabe? If it's the Sunnis that are leading the resistance (I don't know if this is true), and if one of the reasons for their resistance was removed (foreign invaders), wouldn't that help to somewhat defuse the volatile situation? When Saddam was in charge, why weren't Shia's doing suicide bombing?

I'm not really a political person, so these questions might be naive. But are we sure the situation would worsen if America withdrew?

what about the reports a few years ago of americans putting explosives into unsuspecting iraqi's cars at check points and sending them down the road to blow up at the next check point? the reason we found out about some of these is either it didn't explode or iraqi police found them before the left the check point. how many did we not find out about and they were blamed on the imaginary al qaeda in iraq? or what about those british soldiers who dressed up as arabs and shot up a bunch of ppl, were arrested by iraqi's police then the brits broke them out of jail in a raid? seems almost like someone is trying to perpetuate a al qaeda myth eh? hmmm

i'm sure if america left iraq things would stabilize quite a bit....