Why did he call him "Ben Na'avas ha'Mardus"?
Rashi #1: Na refers to motion, a woman who wanders around. Just like Za'avah comes from Za, Na'avah comes from Na. The Sov is due to the Semichus to Mardus. Mardus is one proper to be castigated and punished.
Rashi #2: After Pilegesh b'Giv'ah, each of 200 remaining Bnei Binyamin were told to grab a girl from the vineyard. Sha'ul was too shy to grab one; Yehonason's mother chased him. It says Na'avas due to the vineyard - Na'avah is a winepress (Avodah Zarah 54b), and Yetufun Navahi b'Chemar is [part of] the Targum of "Chachlili Einayim mi'Yayin" (Bereishis 49:12).
Radak: Na'avas refers to his mother. The root is Avah (sin). You get this from your mother, who rebels against my will. So you rebel against what I want, and love what I hate.
Radak: Yonason translates Na'avas to be a noun (a disobedient woman); rebellion [of such a person is harsh]. Abarvenel - according to the Targum, perhaps this refers to David. He is from a disgraceful family. 1
Malbim: A slave rebels against his master to cast off his yoke, or to rule in place of him. A son would not rebel against his father to make a stranger king in place of him! If this happens, it is rebellion of lunatic and perverted intellect. You are a crazy rebel!
Yishai used to teach to a 600,000 men, or he was appointed over 600,000 (refer to 17:12:4:3 and the note there)! Perhaps Sha'ul accepted Do'eg's claim that they are Pesulim, for they descend from Rus ha'Mo'avis! Even though Avner asked in the Beis Midrash, and they were Machshir (refer to 17:56:1:1 and the note there.), Do'eg was "Abir ha'Ro'im", i.e. Av Beis Din (Rashi, 21:8). Also, amidst hatred, Sha'ul accepted Do'eg's opinion. (PF)
Why did he say "l'Ben Yishai"?
Radak #1: The Lamed is superfluous. We find like this - "Hargu l'Avner (2:3:30).
Radak #2: The Lamed is in place of a Beis. We find like this - "Lifneichem le'Charev" (Vayikra 26:7).
Why did Sha'ul get angry now? What did Yehonason do?
Abarvenel (26): Sha'ul called him Ben Yishai, and not by his name, to diminish his importance (Radak 22:8 - to disgrace him. 1 ) Yehonason called him David.
Abarvenel (30): He gave to David permission to go, and escape Sha'ul.
Rashi (Tehilim 4:3): You disgrace me, calling me Ben Yishai - I do not have a name [of my own]?! (We find that Zimri and others who scoffed at Moshe called him 'Ben Amram', and so Kalev called him when he wanted people to think that he will debase him (Sanhedrin 89a, Rashi (Devarim 1:12 and Bamidbar 13:30, citing Sifri Devarim 12 and Sotah 35a). He was called so also when they admired him (Rashi Shemos 14:4 citing Mechilta Beshalach 1), and even Hashem called him so (Shabbos 89a)! - PF)
What is the shame to [him and] his mother's Ervah?
Radak: People who hear that you love the one whom I hate, for you are not my son. This is shameful for you, and for your mother's Ervah, for people will say that she was Mezanah.
Abarvenel: I will be king as long as I live. After I die, David will rule in your lifetime and your mother's 1 . It will be disgraceful, that kingship was removed from you!
Malbim: You remove kingship from her seed to a stranger.
He did not explain why he mentioned "Ervas Imecha." (PF)
QUESTIONS ON RASHI
Rashi writes that "Na'avas" hints to Yehonason's mother, who chased Sha'ul. What is the relevance to here?
Shir Ma'on (Parshas Metzora): A Kalah who is modest in her father-in-law's house merits that kings and Nevi'im will descend from her (Megilah 10b). Since you prefer that Ben Yishai be king, this disgraces your mother's Ervah; she was not modest, and chased me. If not for this, one could have said that she intended l'Shem Shamayim, for Sha'ul was a big Tzadik (refer to 13:1:1:1). Now that you will not be king, this shows that she was not l'Shem Shamayim. Really, she was l'Shem Shamayim; she did not merit that her son be king due to Sha'ul's sin with Amalek.
