Why does it say that Yo'av and the army came? Verse 22 already said so!
Kli Yakar: Initially, Yo'av came to David to honor him. In front of David, no one mentioned that Avner came and left. Yo'av went home with the army (not Avdei David) to enjoy the spoils, and fools told him about Avner. Yo'av returned to David to discuss this.
Malbim (22, 23): The Navi gives three reasons why Yo'av killed Avner. (a) They brought much spoils, and his spirit was lifted to do as he desires. (b) Avner was not with David. If he were, David would have protested. Also, since Avner left, this gave Yo'av a pretext against him. (c) Yo'av came with the entire army. Not one man died. Also this gave clout to Yo'av to do as his heart desires.
What is the significance of "va'Yelech b'Shalom"?
Tanchuma Shoftim 19: When it says that someone goes l'Shalom (like Yisro blessed Moshe (Shemos 4:18), he goes and returns. When it says b'Shalom (Avner and Avshalom - 15:9), he does not return.
