Well, maybe not murderous, but the concept of an evil/sel-serving Avatar doesn't seem like a bad one. Not very original, but not that bad. When you consider the fact that the Avatar is meant to be the balance to set things right between the nations and the spirit world, what would happen when an Avatar destroys that balance themself?
It could easily be explained that the Avatars are inheriently good, but meh. Seeing as how there have apprently been hundreds of Avatars, one would think that at least one of them was atleast a little bit jerkish. Maybe it's even a part of the plot of the second season, maybe the spirit of an evil Avatar is causing trouble.