Some people will go to great lengths to manipulate outcomes when they've turned their backs on the Lord.  This was true of Saul as he broke the law of the Lord to consult a witch in the middle of the night.  This incredibly strange encounter shouldn't be misconstrued as an actual medium because the witch was quite surprised when Samuel actually appeared.  More than likely, God sent Samuel to give one more important message to Saul: your time is up.  Saul had moved so far away from what God desired as a king, and he lost his privilege for leading.  What Saul did, though, isn't altogether different than what we do when we turn our backs on God, too.  We try to manipulate circumstances and even violate moral values simply to accomplish what we want.  There's such a stark contrast between David and Saul that it's no surprise God approved of David taking over the throne.