As a player, I have several times run evil characters in an otherwise all-good party. The key to doing so is to remember that the game is supposed to be fun for everyone. It helps a lot if you are willing to play a redeemable evil character (i.e., a character who begins play as evil, but whom you want to have "see the light" when the opportunity presents itself). This can provide great role-playing moments for everyone.
One example: I once played an evil wizard in an adventure where slavers stole women and children from the town he lived in. The character was not a generally nice guy, but he was lawful evil, and he viewed targetting women and children as ignoble. So, he went along with the group, provided invaluable aid, and was willing to resort to unsavory methods against the slavers. He was also scornful of the moral position of the group as a whole, viewing their "good" as only so many words. At the end of the day, the group was successful.
The party returned at night. As the "good" characters woke the Mayor to collect their reward, the "evil" character eschewed all reward -- although he viewed women and children as "helpless" and "lesser beings" (something designed to give the female PCs a chance to show him the error of his ways), defending them was simply necessary for a lawful society to survive, and nothing to either reward or punish. He turned his back on the "good" characters, point proven, and the adventure ended. It was fun for everyone.
The evil "I'm in it to betray everyone" PC is no fun. The evil-but-noble PC is a lot of fun.
Usually, as a DM, I'm willing to allow evil PCs who have some form of redeeming angle to their characters. I also look at the player; i.e., do I think the player is aiming to dominate the game?
RC