Why must Yo'av and everyone tear and mourn over Avner? Only his relatives must do so!
Radak: Even though Yo'av killed him, he must tear over him and mourn, for he killed him improperly.
Malbim: All must regret the murder, and know that he was killed improperly.
It says (Sanhedrin 20a) that a king does not follow the bier, even for a relative!
Radak, Malbim, from Sanhedrin 20a: David did so people will know that he was not killed through David's counsel 1 , like it says below (verse 37).
Perhaps David merely pretended to be pained! Malbim - the Gemara says that he went among the men and among the women. One may go among women only if he truly mourns. If not, it is forbidden due to thoughts of sin!
