3-3 is the confrontation with the one who controlled all this, and a battle of freedom against manipulation. The Old Monk hoped the golden robe was a way to escape his mortality, yet the garment simply uses him for its own interests before leaving him to the realities of his body. Then it takes on another host looking to restore their humanity, tempting them with its power. But Slayer of Demons has benefited from escaping false hope's manacles, and so we see him/her triumph over a soul imprisoned by it.
One of the more interesting details about Tower of Latira and
the Old Monk is that the golden garb is actually the archdemon and not the host it takes. The cutscene when it takes on another host is very telling of this. After the player (the slayer of demons) kills the maneaters and approach the Old Monk through the stairs, the Old Monk summons a powerful red phantom to protect him from the player now that he's dying and too fragile to protect himself because the player took down the artificial heart before in 3-2 that gives him lifeforce and blocks the path to the maneaters and the monk's throne. Now that the he's slowly dying and summons a new powerful red phantom to protect him, the golden garb realizes that the summoned red phantom is a better host for its survival and leaves the Old Monk, who almost immediately dies when the golden garb that kept him alive left him.
God, I love Souls lore.