Faulkner's title is an allusion to the biblical story in Samuel 2:13-18 in which Amnon, King David's oldest son, has incestuous relations (rape) with Tamar, his sister.  Their brother, Absalom, outraged by his brother's incestuous action, arranges for the killing of Amnon, after which Absalom flees to Jerusalem, and ultimately returns as king.  Eventually, Absalom and his father, David, go to war with each other, and David's general, Joab, kills Absalom (against David's wishes).   The title Absalom, Absalom! comes directly from King David's reaction when he hears that Joab has killed Absalom:

O my son Absalom, my son, my son, my son Absalom! would to God I had died for thee, O Absalom, my son, my son. (Samuel 18:32-33 KJV)

The character whose actions and life most closely parallel that of Absalom is Henry Sutpen, the son of Thomas Sutpen.  As Clytie recounts:

I saw Judith's marriage forbidden. . . .I saw Ellen die with only me, a child, to turn to. . . .I saw Henry repudiate his home and his birthright and then return and practically fling the bloody corpse of his sister's sweetheart at the hem of her wedding gown. . . . (1.12)

Clytie is, of course, referring to Henry's violent reaction to his father's statement that Charles cannot marry Judith (because Charles and Judith are half brother and sister, which Thomas fails to tell Henry), and Henry's subsequent violent departure from his home and father.

Although Henry and Charles develop a true, if sexually ambiguous, friendship, when Henry discovers that Charles is of mixed race, Henry eventually prevents the marriage by killing Charles at the gate of "Sutpen's Hundred," Thomas Sutpen's estate, and escaping to parts unknown for several years.  When Henry returns, he essentially hides himself away until he is inadvertently killed in a fire set  by Clytie, his own half sister, who is trying to protect him.

The biblical tale of Absalom is not a perfect match in Faulkner's interpretation, but the essential elements of a son's rebellion against his father are present and, more important, the tragic consequences of the rebellion are played out in the novel.