What kind of Evil game?
I've run 2-4 session evil games where anything went, PKing was attempted numerous times and the whole group had a blast trying to betray, undermine, backstab (figuratively and literally) and destroy each other while wreaking havoc. The last session ended with the wizard casting crushing depression on a group of children while the barbarian used one of them as a club against the others, then the cleric cloudkilled them all, leading to an arrest, a bungled escape attempt and all 4 party members being hanged, one before the rest because he tried to backflip out of his noose and just broke his neck instead.
But that game was cartoonish and meant to get that stuff out of the group's system.
I played another game with a LE fighter and a LG cleric who often butted heads, but the LE fighter was savvy enough to cover his evil deeds and minimize them and the LG cleric was able to turn just enough of a blind eye and would take 10 on his sense motive checks to fail against the fighter's bluff. The campaign went on for months, there was some incredibly interesting roleplaying and noone killed each other. However, I was prepared for the day the Fighter slipped and him and the Cleric went toe to toe, and so was the rest of the group. It had been discussed and it's nigh inevitability made many situations have more suspense and drama.
I think you just have to have a discussion with your group and decide what the consensus is. If someone seems pretty distressed about it, ban it. But if the group acknowledges that sometimes evil rolls that way, cool.
Also, I disallow PKing in sessions with Newbies. PKing is a master class kind of thing, IMHO.