What does this Mizmor discuss?
Malbim: It has two parts. Until verse 8 is a Tefilah to be saved; the rest discusses that he did not kill Sha'ul; he overcame his Yetzer, and this was glory for him, to the point that Sha'ul recognized his virtue and integrity.
Why does it say "Al Tashches"?
Rashi: So David called this Mizmor, for he was close to death.
Radak: We do not know the reasons for the names of the Mizmorim. The three coming Mizmorim all discuss David with Sha'ul, and all begin Al Tashches. Perhaps it is a Tefilah that Hashem not destroy him in Sha'ul's hand. David said this when he and his men were in the cave, and Sha'ul came to relieve himself.
Malbim: Also Mizmor 142 is about when he was in the cave. That discusses only his affliction; here it discusses overcoming his Yetzer ha'Ra not to be Mashchis (kill) Sha'ul, so it is called Al Tashches. In Shmuel (I, 26:9) David used this expression ("Al Tashchisehu").
