I don't think there should be a mandatory negative trait but think traits should be redone. Traits should impact autonomy and dialogue with other sims. For example, a good sim can get moralistic with other sims. They should have interactions with evil to be able to save them and bring them back into the light. Just as evil should be able to corrupt, manipulate, and destroy others. For example, Vlad should be able to go to a club see Geffory Landgraab cheating on Nancy with yet another random sim and he should be able to expose Geoffry's indiscretions or be able to blackmail Geoffry because he is evil. On the other hand, Geoffry should be able to save his son Malcolm from the dark path he is from being a good sim.