The aftermath of a rebellion often brings complex emotions, but a king's public reaction to the death of a traitorous son carries heavy legal and political weight. When news reaches the army commander that King David is not merely crying but actively mourning the death of Absalom, it exposes a deep and dangerous tension between the two men.
Joab fully expected David to express sadness and shed tears. It is completely natural for a father to weep over the loss of his child, regardless of the circumstances. However, the commander is both surprised and deeply angered by the king's choice to engage in formal mourning.
According to the law, formal mourning is not observed for individuals executed by a court or those lawfully killed for leading a rebellion. By choosing to officially mourn, David makes a profound legal statement. His actions declare that Absalom's death was not a lawful execution of a rebel, but an unjustified killing. This public display leads to a severe conclusion. It implies that in the king's eyes, Joab is no longer seen as a loyal general who suppressed a revolt, but rather as a murderer [מלבי״ם].