John Gill Commentary 2 Samuel 19

And it was told Joab
When returned to Mahanaim, or on his way to it; perhaps by the messengers he sent; and this report he had before he saw the king:

behold, the king weeps and mourns for Absalom ;
for the death of him, instead of rejoicing at the victory obtained, and the deliverance from his enemies.

And the victory that day was [turned] into mourning unto all
the people
They also mourned too, instead of expressing joy upon the occasion:

for the people heard say that day how the king was grieved for his
son ;
this report was spread among them, which damped their joy, and hindered them from giving any tokens of it, as were usual at such times.

And the people got them by stealth that day into the city
Did not march into it in companies, in a public and triumphant manner, as conquerors used to do; but entered in a private manner, one by one, or a very few together, not caring to be seen or known, at least by the king, as fearing they had incurred his displeasure: but

as people being ashamed steal away when they flee in battle ;
as if they had been conquered, and not conquerors; nay, had acted a cowardly part, and ran away; and so cared not to be seen, lest they should be reproached, or suffer for their cowardice.

But the king covered his face
And would not see his generals, and thank them for their services: but wrapped himself in his mantle, after the manner of mourners:

and the king cried with a loud voice, O my son Absalom! O Absalom, my
son, my son !
and this was some time, perhaps some days after he had received the news of his death, since Joab and the army were returned from the battle; had it been a sudden start of passion, upon first hearing thee news, and had continued a few hours, it would have been more excusable;

But to continue some days, as it is very probable it did, was very unbecoming.

And Joab came into the house to the king
For by this time he was removed from the chamber over the gate to his own dwelling house or palace, where he continued the same doleful ditty as at first:

and said, you have shamed this day the faces of all your servants ;
they cannot lift up their heads, and look any in the face, nor one another; but behave as if they had committed some very great fault, in fighting with the rebels, and beating them:

which this day have saved your life, and the lives of your sons, and of
your daughters, and the lives of your wives, and the lives of your
concubines ;
which in all probability would have been taken away if the victory had been on the side of Absalom.