What is the meaning of "Hayu Ven Sha'ul"?
Rashi: It is as if says Hayu l'Ven Sha'ul, just like "la'Mo'ed Asher Shmuel" (1:13:8) is like la'Mo'ed Asher li'Shmuel.
Radak citing Targum Yonasan: They were with Ben Sha'ul.
Radak: "Sarei" refers to Gedudim, and also to Ben Sha'ul (they were officers of him).
Radak citing Menachem: "Hayu" is like Shabru, like "Nihyeisi v'Nechelisi" (Daniel 8:27). I.e. they killed him. This was not Ish Boshes, rather, a different son of Sha'ul, in Sha'ul's lifetime or after his death.
Why does it say that they were officers of Ven Sha'ul?
Radak: This explains why the household did not beware of them when they entered his room; they thought that they came to talk to him.
Why does it say "Gam Be'eros"?
Radak: Perhaps it was on the border, and people thought that it is not from Binyamin. The verse teaches that also it was from Binyamin. The ones who finished off Beis Sha'ul and his kingship were from his Shevet!
What is the meaning of "Techashev Al Binyamin"?
Radak: This is like El Binyamin. Also "va'Tispalel Al Hashem" (1:1:10) is like El Hashem, "Al Shafan ha'Sofer" (Melachim II, 22:8) is like El.
