Absalom is a random encounter one will likely to encounter early in the game. To me on many playthroughs it always occured before meeting the railroad.

The usual encounter location of absolam is "encounter point near hangman's alley", "encounter point near the broken house between corvega and college square", "encounter point near Layton Towers".

Absalom has the similar appearance as Deacon, both wear sunglasses, use a scoped long barrel hunting rifle. And he gives no exp reward upon death. ( a hint that player didn't actual "kill" him )

Also when absalom dies, even if he get his head blowed off from a headshot, the player can still hear his dying voice of "Well that's never happened before" ( another hint that the player didn't actual "kill" him )

Deacon knew the survivor's abilities and he wanted to get rid of the player since it would be detrimental for the railroad if the player sides with other factions. That's why he attempted to assassinate ss as absalom. And it failed, that's why Deacon resorted to convince the player to join and fight for railroad's cause, vouch for the player, go as far as to lie to Desdemona to make sure the player joins the railroad, and even to the point gives the player a badass pistol to further fortify player's relationship with the railroad.