I don't try to "moralize" with groups as much as I present evil intentions as just that: fucking evil. Sometimes all it takes to get players (and their adventurer characters) to do the right thing is for them to have a pleasant interaction with a character then see that character threatened by someone else.

Sometimes I've done classic Faustian bargain offers to players, where the villain offers (and really means it) power and the like in return for service, but it lightens my heart that more often than not, the players will say, basically, "no, fuck you. We saw what you did with our goblin friends back in the day!"