In all seriousness, it cannot be overstated how much of a dick move this was:

-Plays up being a bad ass warrior like Jack.
-Invents sob story to create illusion of having a common enemy.
-Travels across incredibly hostile area with Jack.
-Saves Jack's life technically three times.
-Wins Jack's trust.
-Potentially sparks romantic interest.
-Kills giant guardian just to remove final obstacle before the jewel that trigger time portals.
-Destroys jewel as Jack expresses extreme relief that his quest is finally over.
-Laughs in Jack's face as the woman he was starting to fall for.
-Turns into his true form as Jack begs for an answer.
-Mocks Jack for his naivety and reminds him that he's basically alone in limbo.
-Flies away, leaving Jack to literally scream with rage and fall to his knees in grief.

Like… holy shit… I don't think I can count on one hand the examples in fiction that I know of where a villain went this far out of their way just to break the morale of the hero.