Why does it say "Ben Ish"?
Radak: He was the son of a great man who fears Hashem. "Anashim" always refers to great people.
Malbim: Amasa was called by his father's name. Yo'av's father was not known, so he was called by the name of his mother, David's sister.
Why does it give the name of both Amasa's parents, and for Yo'av, only his mother's name?
Malbim: Refer to 17:25:1:2.
Here he is called Yisra ha'Yisraeli. In Divrei ha'Yamim I, 2:17, he is called "Yeser ha'Yishmaeli!"!
Radak: He was a Yisrael, but he lived in Eretz Yishmael. My father said that when he lived in Eretz Yishmael, he was called Yisraeli. When he lived in Eretz Yisrael, he was called Yishmaeli.
Radak citing Yevamos 77a: He is called Yishmaeli because when Sha'ul wanted to disqualify David for kingship because he descends from Rus ha'Mo'aviyah, Yisra girded his sword like a Yishmaeli and said 'whoever does not accept my tradition from Shmuel ha'Navi's Beis Din will be pierced with a sword. An Amoni and a Mo'avi are forbidden, but not an Amonis or Mo'avis.' The Isur is "because they did not go out to greet you [with bread and water]" (Devarim 23:5). This reason does not apply to women, for it is normal for men to go out to greet, but not for women 1 .
Some explain that Mo'av is forbidden only due to hiring Bil'am; only Amon failed to offer us food when we passed by them (refer to Devarim 23:5:151:1)! Perhaps it is clear that Amonis and Mo'avis have the same law, so we can learn Rus' status from the law of Amonis. - PF)
Why does it say "Asher Ba El Avigal", and not Ish Avigal?
Radak: Perhaps he had Bi'ah with her and fathered Amasa before he married her.
Why is she called Bas Nachash? Divrei ha'Yamim I 2:13-16 says that she was Bas Yishai!
Rashi citing Bava Basra 17a: Yishai, the father of David, is called Nachash. He died without sin, due only to the [sin of Etz ha'Da'as, which came via the counsel of the] snake.
Radak: Nachash is Yishai. He had two names, like many people.
Radak citing his father: He is called Nachash because Tzif'onim (a type of snake) came from him and killed nations - "miShoresh Nachash Yatza Tzefa" (Yeshayah 14:29) - Chizkiyah comes from Shoresh Nachash, i.e. Yishai.
