To sum it all up:
The insurgents (Muslim extremists with former Baathists and foreign fighters) want to start a civil war.
There is no real ethnic tension in Iraq and in order to provoke a civil war, the insurgents have:
1) bombed Christian churches
2) bombed Shia religios events
3) assassinated Shia leaders
4) attempted to assassinate
Ayatollah Sistani
If they do manage to kill Ayatollah Sistani, then a civil war will erupt no doubt and it'll be the bloodiest thing for this generation.