John Gill Commentary 2 Samuel 18:33

"And the king was much moved, and went up to the chamber over the gate, and wept: and as he went, thus he said, O my son Absalom, my son, my son Absalom! would I had died for thee, O Absalom, my son, my son!" — 2 Samuel 18:33 (ASV)

And the king was much moved
His affections were moved, his passions were stirred up; he was greatly troubled, distressed, and grieved:

and went up to the chamber over the gate, and wept ;
got out of sight and company as soon as he could; as his own dwelling was at some distance, he made haste to the chamber in the watchtower, over the gate of the city, where the watchman was, to vent his grief; and could not suppress it till he got thither:

and as he went ;
up the stairs to the chamber:

thus he said, O my son Absalom! my son, my son Absalom !
which repetition expresses the vehemence of his affections, and how inconsolable he was on account of his son's death:

would God I had died for thee, O Absalom, my son, my son !
some think he said this on account of his eternal state, being satisfied of his own; but it may be it was only the effect of natural affection, indulged to too great a degree, and unbecoming so good a man in such a case; the Targum is, ``I wish I had died for thee, and thou hadst remained this day.''