1)
How did Sha'ul know that David will reign?
1.
Rashi: Hashem saves you from my hand.
2.
Rashi (from Midrash Tehilim 57): (When Shmuel told Sha'ul that Hashem will give his kingship to someone else, Sha'ul asked whom.) Shmuel said, the one who will tear your robe, he will rule in place of you.
3.
Radak: Perhaps he heard that Shmuel anointed David (and this sufficed, even without the Siman - refer to 24:20:1:2).
4.
Malbim: It is because you suppressed your Yetzer to such a level. One who rules over his Yetzer is proper for kingship.
2)
Why did he add "v'Kamah b'Yadecha"?
1.
Radak, Malbim: It will last with you, unlike me, from whom kingship was torn.
