The "good cop" is the one trying to smile and play nice as a manipulation tactic, presenting a "nice, reasonable option" that "just wants to help you" that's contrasted with a bloodthirsty psychopath that's threatening/actively torturing you.

The "good cop" doesn't actually do anything good, they're just polite and suggest they might do something good if you just do what they want, and focus on how much better an option they are than the "bad cop" who's openly antagonistic and trying to harm you. Both actors in the scenario are bad, their descriptor is just about the role they're playing in the con.