1 Samuel 18-19

Saul is insanely jealous of David. David has more success in battle, he is loved by more people, and he keeps on getting more and more famous. So what does Saul try to do? Kill him. Actually pin him to the wall with a spear. But David always gets away "because the Lord was with him."
One thing I don't get is why whenever Saul does try to kill David "an evil spirit from the Lord [comes] on Saul." What is up with that?
In the last part of ch.19 David escapes and goes to be with Samuel. Everyone that Saul sends to kill David, and eventually himself ends of prophesying... Why?