THEORY: Vladimir and Estragon were "slaves" in their past life, and are now slaves to each other. Estragon threw himself into the Rhone river, committing suicide. Vladimir killed himself soon after, now they are both in hell (purgatory?). The boy is different from them, possibly representing a prophet, or hope. Lucky and Pozzo are interchangeably master and slave.
TEXT SUPPORT FOR OUR THEORY: Pozzo, about Lucky, “He used to be so kind...my good angel...and now...he’s killing me.” (pg. 23) Although Pozzo bosses Lucky around, Lucky is a master in his own way.
Estragon committed suicide, “Do you remember the day I threw myself into the Rhone? Vladimir: We were grape harvesting. Estragon: You fished me out. Vladimir: That’s all dead and buried. Estragon: My clothes dried in the sun.” (35) The boy is the one who says, “Mr. Godot told me to tell you he won’t come this evening but surely to-morrow.” (34)