Why did he say that they were "Tzadikim v'Tovim Mimenu"?
Radak #1: They did not spill innocent blood like he did.
Radak #2: Perhaps they had attributes that he did not have.
Radak citing Sanhedrin 49a: They were Tzadikim in the mouth and heart and did not do [a verbal command of the king contrary to Torah]. Sha'ul commanded "la'Ratzim ha'Nitzavim Alav" (Shmuel I, 22:17) to kill Kohanei Nov. The Yerushalmi (Sanhedrin 10:2) says that Avner and Amasa were there; they refused to kill them. Yo'av fulfilled a written command [to cause Uriyah's death]. Amasa and Avner were Tovim Mimenu - they expounded 'Ach' and 'Rak' to exclude a command of the king contrary to Torah; one must not obey it - "Kol Asher Yamreh Es Picha... Rak Chazak ve'Ematz" (Yehoshua 1:18). 1
Sanhedrin 49a: Also, when David sent Amasa to gather Yehudah, they were learning, therefore he waited until they finished.
Why did he say "v'Avi David Lo Yada"?
Radak: One might have thought that David knew about (and authorized) the murder of Avner, who was from Beis Sha'ul. Through David's lamentation and fast over his death, people realized that David did not know. One might have thought that David knew about (and authorized) the murder of Amasa, who had supported Avshalom. Here Shlomo taught that he did not; David had told Amasa to gather Yehudah and return 1 (Shmuel II, 20:4)!
Rashi (Shmuel II, 20:4): David wanted him to chase after Sheva ben Bichri. Radak - he wanted to appoint Amasa to be the head general.
