Evil campaigns that I've played in tend to fall apart because the characters wouldn't group together. Everyone was stealing from each other and wanted to do their own heinous things and get full credit and all that.
Or, in games that I've ran, they play stupid evil and get chased down by a posse.
Me - "OK, so there are four people in your party and you are all first level. There are 500 farmers in this town, and are firends with neighboring towns, and you have no mounts. Are you sure you want to rape the little girl's butter churn?"
Blumcorps (bloom'-core) - "I'm already on it man...mmm, buttery"
The good thing about that example was that the party totally helped the town kill Blumcorps so they would not be perceived as being associated with him.