The harsh rebuke delivered by the army commander exposes a painful collision between the personal grief of a father and the public duties of a king. By mourning his rebellious son, Absalom, David displays what his weary soldiers perceive as deep ingratitude toward the men who risked their lives to save the monarchy. David's behavior reflects a tragic inversion of loyalty: loving his enemies and hating his friends. By grieving over the death of his adversary, he essentially projects anger and resentment toward the loyalists who saved his own life [מצודת דוד, ביאור שטיינזלץ].
The danger of this misplaced affection goes beyond hurt feelings. Had Absalom survived, he would have inevitably relied on David's enemies to exact revenge against the king's loyalists. Therefore, David's desire for his son to live would have actively empowered his foes and endangered his supporters [מלבי״ם]. Furthermore, the king's public display of sorrow sends a devastating message to his troops. While he does not say it directly, his behavior broadcasts that his officers and servants mean absolutely nothing to him, as though they do not even exist [רד״ק, מצודת דוד]. A leader is expected to place the welfare of the public and the officers who uphold his kingdom above his private love for his son. The public mourning serves as proof that David's priorities have become entirely warped [מלבי״ם].
This leads to the commander's harsh and painful conclusion: David would have gladly accepted a conditional reality where his son lived and his entire army perished [רד״ק]. The logic behind this severe accusation rests on the absolute nature of the conflict. In the war against Absalom, mutual survival was impossible; one faction had to fall. Because David is so deeply distressed by his own army's triumph, he reveals that he would have willingly embraced the opposite outcome. He makes it clear that a victory for Absalom, even at the cost of the lives of all the king's loyal subjects, would have been his preferred result [מלבי״ם, מצודת דוד, ביאור שטיינזלץ].