It goes like this: We all know Duncan was arrested, correct?
We all know he started acting nicer from being happily with Gwen, right?
Well, imagine how he must feel right now in prison, thinking about every wrong thing he's done in life including the realization of what he did to Gwen. All he can do now is think about how great she was, and how much he misses her. He didn't mean to take a step in the wrong direction, and that's what Gwen probably helped him through. Gwen changed the kid from being a bad-boy with an attitude to a softie because of how happy he really was with her. All he can do from his prison cell, is hope she gets everything her heart desires.

(If anyone is wondering, I drew the pictures used.)
(Also, I CRIED SO MANY TIMES WHILE MAKING THIS GAAAAH :'I SO, WARNING: MAY CAUSE TEARS.)