He already requested Hashem to guard him (2). Why is this repeated?
Radak: After telling their evils, he prays to be guarded from them.
Malbim: Before he spoke generally, about those who seek to harm without benefit or for benefit. Now he specifically requests salvation from the Zifim, who did not seek benefit 1 , and from Sha'ul, who wanted to harm David to benefit his own kingship.
Teshuvas Radvaz (5, Leshonos ha'Rambam 1): The Zifim told Sha'ul in order that he would feel gratitude towards them, like is explicit in the verses.
What is "Lidchos Pe'amai"?
Radak: It is to fall into Sha'ul's hand.
Malbim: To make me fall into the pit that they (the Zifim and Sha'ul) prepared for me.
