1)
Sha'ul promised his daughter to the one who will kill Galyus. How could he now stipulate that David must fight to marry her?
1.
Malbim: I can fulfill my promise through my younger daughter; you will need to wait until the older gets married. If you want the older one now, I can stipulate.
2)
Why did Sha'ul prefer that Pelishtim strike David?
1.
Rashi: He did not want to be punished.
