I've had the trolls pegged as evil since the first viewing for one specific reason: "Only an act of true love can thaw a frozen heart". That was what sent Ann

I've had the trolls pegged as evil since the first viewing for one specific reason: "Only an act of true love can thaw a frozen heart". That was what sent Anna off to find Hans, but also the key for Elsa. Love is at the heart of her control of her powers, not fear. The trolls knew, but instead of teaching her this and helping her learn how to control her powers, they taught her to fear them.