What is the meaning of "Yacholu"?
Rashi: [Liability for] the blood of Avner, mentioned at the end of the previous verse, should fall on Yo'av's head. Radak - this is like "Al Rosh Resha'im Yachul" (Yirmeyah 23:19).
Yo'av sinned. Why did David curse his father's house?
Radak: He cursed only those who helped him. This is like Targum Onkelos of "uv'Mishpachto" (Vayikra 20:5, regarding Molech) - uvi'S'adohi.
Malbim (30): Also Avishai (Yo'av's brother) helped to kill him (verse 30).
What is "Machazik ba'Pelech"?
Rashi: He leans on a stick due to illness of his legs.
Radak: It is a fool.
Malbim: It is one who holds a knitting needle (for his livelihood).
Were these curses fulfilled?
Sanhedrin 48b: When Shlomo commanded to kill Yo'av (like David commanded him), Yo'av said that if so, he must accept the curses on his seed 1 . Shlomo accepted, and they were fulfilled on his seed. 2
Maharsha (48b): Since he was killed for his sin, he was cursed without reason, therefore it reverts back to the one who cursed. (The curses were for killing Avner. The Sanhedrin exempted Yo'av for Avner, and convicted him for killing Amasa and supporting Adoniyahu's rebellion (49a)! - PF) Radak - David cursed him improperly, for he intended to kill him. (Why can he not do both? Even if Beis Din normally gives only one punishment, a king can punish unlike Torah law. When David heard about the Ashir who stole the Ani's lamb, he said that he will die and pay four-fold! (12:5-6 - PF)!
Radak citing Sanhedrin 48b: Rechav'am was a Zav. Uziyahu was a Metzora. Asa needed a cane. Yoshiyahu was killed by the sword. Yechonyah lacked bread.
Why did David give these curses, and curse also Yo'av's seed?
Radak citing Yerushalmi Kidushin 1:7, Malbim 1 citing Eduyos 2:9: A righteous person merits five things for his children - beauty, strength, wealth, Chachmah and long life. The opposite applies to a bad generation with evil parents. We learn from Yo'av; David cursed him, for his sin was clarified. A Zav is weak, the opposite of strength. A Metzora is the opposite of beauty - "v'Sipach 2 Hashem Kadkod Bnos Tziyon", "Ki Sachas Yofi" (Yeshayah 3:17, 24). 'Holding a stick' is a fool; this is the opposite of Chachmah. Being killed by the sword is the opposite of long life. Lacking bread is the opposite of wealth. David knew that Yo'av's descendants will be Resha'im, therefore he cursed them with these five.
Malbim explains like Radak, but he says that Machazik ba'Pelech is one who knows to earn money only through weaving; women's Chachmah is only ba'Pelech (with a knitting needle - Yuma 66b).
Shabbos 62b: This teaches that Tzara'as sprouted in them. "V'Sipach" is like "vela'Sapachas" (Vayikra 14:56).
