I cant believe with this many commenter's in here EVERYONE MISSED THE POINT!
Just hear me out. I keep reading how they go on a killing spree finally happy together, the end. NO! Here's the key you missed ; both alternate singing "and if it not enough, I'll try again, and again, and again, OVER AND OVER AGAIN!"
This is HELL! Their own personal hell. And because of their horrible acts in life, they are doomed to repeat the same cycle they chose in life. Forever they will almost grasp happily ever after but be denied by their own selfishness and lack of faith in each other. Then intense love denied fuels their hate and they destroy one another. Viciously. Over and over again. For eternity.
Over and over again.