What is the meaning of "Asher Yevakesh Es Nafshi Yevakesh Es Nafshecha"?
Rashi citing Targum Yonasan: The one [Sha'ul] who seeks to kill me, he seeks to kill you.
Rashi: The One who loves me, He will love you. The One who guards my soul, He will guard your soul. Yevakesh is sometimes for the good - "Yesharim Yevakshu Nafsho" (Mishlei 29:10).
Radak: My soul will seek [the downfall] of one who seeks [to harm] your soul.
Malbim: I am in more danger than you. The one who seeks [to kill us], primarily he seeks to kill me. You are only secondary to me. Therefore, you should not fear. 1
This does not explain why Evyasar should stay with David. He is in more than danger if he is near Sha'ul's primary target! (PF)
What is the meaning of "Mishmeres Atah Imadi"?
Radak: I will guard you like I guard myself.
Kli Yakar: You are a Chacham, and a great man's son. You are a protection for me - surely I will not harm you!
Malbim: For being saved, you are primary. 1 You are a Kohen, and do Hashem's Avodah. Along with you, He will guard also me, for I am secondary to you. Therefore, you should not fear.
Was not David guaranteed to be saved, due to Hashem's promise [that David will be king] "Netzach Yisrael Lo Yeshaker