Actually Lazarus, i think there are scriptures to back this.
Unfortunately im right now at work and am too tired to go search better, so im just pointing out some things and one verse from memory.
Jesus: "But i say, if you even look lustily upon a woman, you have already committed adultery"
This and some other verses in Bible seem to indicate that we have to be very careful about what we speak, that even vain words can be destructive, and also that we should be careful even about our very thoughts!
So taking this in count, i would say there is backing to the idea of not giving player a chance to do evil since that wold be thinking of it.
But if we accept this, we also get to another problem. Should we then not read all of the Bible?
For "if you even think of a woman lusty, you already committed sin" To read and understand that verse, you need to first think of lusting a woman. whops... sin!
What comes to my mind is that heartoffire is bascially right, but that it is the highest goal kind of thing.
What i mean by this is, that Bib,.e clearly tells us "Thou shalt not kill". Yet, God Himself told to get the promised land and kill all the people and even animals from there?
Im not saying it was Gods will, since it wasnt. Gods will was that these people had peacefully moved away from the place, but since that was not an option, God had no other choice but to do it this way.
So while Killing is absolutely not a thing we should do, it can be acceptable choice in some situations. However, we can see that for example David didnt get to build the temple, because he had been stained by the blood of his enemies. Despite the rightfulness of his wars.
I am thinking that maybe hearoffire is right, that in highest goal, we should not let players do evil choices, (nor maybe even read bible from some places, since they would anyway have the understanding even without those places if theyd be living to the highest degree). But because we are living in this non inperfect world, it isw acceptable choice to let player do those evil choices, because it might be necessary for teaching purposes, like it is necessary for bible to tell us about some horrible things and make us think of them for our own good.
Or as it was necessary to kill everyone from promised land, because world isnt perfect.
Just a thought that came to me when i was thinking of this.
For problem with heartoffires comment is, that i can find support to the idea from the bible, but the support i get from bible, would also indicate that i shouldnt be reading all of the bible!
Therefore this thought comes to my mindas the possible solution.