Here's something to consider with Locke's quest (and this may answer questions about other quests as well).
Locke is the quest giver. What this means is he sets the parameters for what constitutes succeeding. Macbeth's argument aside, the quest is not "resolve this situation," much less "resolve this situation in a good, not evil, way." It is "get my money back from the guard."
While helping the guard may very well be the "good" thing to do, with getting the money being "evil," the quest is quite specific in that you must get the money from the guard and return it to Locke.
No one ever said every quest must be just, nor are quests any more than an errand at the behest of the giver.
