why does chillingworth's smile "convey secret and fearful meaning?"

Hester finds out from the mariner that Chillingworth has joined the crew of the ship that will take Hester, Pearl, and Dimmesdale to Europe. Hester glances at Chillingworth and sees his evil smile. Chillingworth will not let Dimmesdale go. He is obsessed with punishing and torturing Dimmesdale, even if it means following him to a different continent. Chillingworth harbors secrets and cruelties to inflict on Dimmesdale that Hester cannot comprehend.