As GM of my last Forgotten Realms campaign, I decided that the end result of the actions of the player characters was that the world as they knew it descended into chaos and barbarism, with every major city in the world (Waterdeep included) burning brightly, and the vast majority of the population falling prey to the wild creatures of the night as they fled.
Then I found out that they were planning to do that anyway with the 4E conversion, and I didn’t feel so evil.
The ability of otherwise innocuous people to do trivially evil things is one of the reason why you can’t target NPCs for attacks in World of Warcraft; as it is, every village you pass by, all the cows, chickens and rabbits are dead. If you could add NPCs to the list, all the little kids would be dead, too…
of course, the town guards might not like that so much, but still… if I could go into Stormwind, kill Billy, and give that little girl her doll back… there’d be a line stretching clear to Ironforge.