Why iraq's people kill soldiers

U.S. and U.K. release iraq's people from suppression.
But iraq's people kill soldiers of U.S. and U.K.
Why?
 
Please teach...

This is all well and good, but the real question at this point is how do we get ourselves out of this mess?  At this point, it's all throwing good money (and men) after bad.  On the other hand, we can't exactly just pull everyone out now, or the place will deteriorate into anarchy faster than you can say "WMD."  Any thoughts?