Just a guess, but killing Saddam might go a long way towards deflating his followers whose personal position in life came from his being in power. If they know he's dead and gone, then they know he can't regain power so there's no point in continuing to oppose the new government. Remember, as in virtually all totalitarian governments, there are quite a few people who prosper. They aren't likely to do as well (or even survive) if the tyrant they support is overthrown.

On the other hand, it's unlikely that killing Saddam will stop the foreign terrorists who've gone to Iraq to attack us. They aren't fighting for Saddam, they're fighting against the infidels. If anything, killing Saddam might even spur them to more attacks like what has happened since his sons were killed. The good part about this is that many of these foreign terrorists are getting killed or captured, so this process is serving to thin the herd (or clean up the gene pool) somewhat.