Malbim: This Mizmor is divided into three. Until verse 7 discusses when Sha'ul cast the spear at him to strike him, and Hashem saved him. After this, until verse 15 discusses when Sha'ul sent guards at night to guard the house [lest he escape] and kill him; he found out, with Hashem's help; he prays that Hashem save him. The last part curses his enemies.
2)
Why does it say "Al Tashches"?
1.
Rashi: So David called this Mizmor, for he was close to death. He requested mercy.
2.
Malbim: Sha'ul should not destroy him.
3)
Why does it say "va'Yishmeru Es ha'Bayis"?
1.
Rashi: This was when Michal told them that David is ill, and she helped him to flee at night.
