I have found in the past that players have a moral compass all of their own if you restrain them just enough to motivate them. That might sound odd, but legends, myths, fairy tales, and fantasy stories often do have very strong moral underpinnings, and I found that players actually relate to that instinctively, though on the day when I'd had enough of hack ^ slash for 'evil' characters (most of whom did not understand evil motives and used the category to avoid restraints on their game behaviour), they did moan and whinge - but the result was a more focused group identity. Acting in the name of 'Good' meant monsters really were adversaries, though I did often introduce elements of decision. A few examples....
1 - The group met a strange patron who remained behind a curtain, asking them to obtain an item of rare significance. The players were curious but warned off. Having completed the quest, they learned they had been working for demon that was trapped on the material plane and needed help to go home. The end justifies the means? They seemed to think so even after the shocking expose at the end of the quest.
2 - The group reach an important city and found it had been razed to the ground, a mysterious being ruling the area as a tyrant. They started searching the runs and I told them a group of evil guards were approaching, led by a gaunt, impossibly tall and thin leader. They hid, not sure of what they were dealing with. Then I told them they heard a baby crying, the mother trying desperately to hush the child. The evil leader heard it too and suggested his guards put the sad creature out of its misery. I have never seen players move so decisively as they did then. To heck with caution, the child must be protected! All very instinctive you see.
3 - The players had come across a certain NPC many times, a noble warrior with ambition, a very violent and dark personality. One player in particular had a long standing feud which was great for role playing. But because of severe political changes, they found this NPC, left to die, hopelessly disabled, a requirement of the overall story. The players didn't gloat, not one of them. A mercy killing took place, and the player who did that was genuinely appalled and sorrowful for the fate of their long time enemy, they were almost respectful when the time came.