Generally I never ever ever EVER ... EVAR allow Evil PCs in my games. Just leads to too many problems. Even if it's not going to lead to problems ... it'll find a way to lead to problems.
With this is rules on PK behavior. I'll go right OOC table-top and tell folks it's not going to fly. I'll explain why, but it's not going to be fun for anybody at the table but the guy doing the stabbation, and it's not going to be fun for him very long.
I actually tried PLAYING an evil character in a recent campaign. It was, I thought, the only way an evil PC could be played in a game without ruining the game. Honestly, I was a wee bit cocky. I had a good back-story, a long-term driving goal that would seamlessly unite him with the party and with a good-aligned group. He was so well tied up that the only way TO role-play him would leave him ACTING almost like a paladin. So basically he was evil in that he didn't particularly LIKE doing good, but did so anyway. In function he was perfect for the campaign ... in mannerisms and role-playing, the most fun I've ever had ... just generally a haughty snotty S.O.B. (he called all the gnome NPCs "halflings" because "all you little people look alike", he generally reffered to everybody in the party not a Drow as "those of the slave races", and hired one of the other PCs as a body-guard). But, little by little, as the campaign progressed along ... the rest of the party started turning evil. The player with the good (most were Neutral in some way) started acting evil as all get out. Eventually the neutral clerics were raising undead to help in combat, contemplating Desecrating pastoral groves to raise MORE undead, and more or less being black as heck.
It got to the point where we all had to generally go OOC and decide to help the GM get the game back on track and headed in the direction he originally intended ... which was pretty heroic, with the party acting as a group of freedom-fighters against an oppressive empire.
While I don't think there was anything wrong with my CHARACTER, or how I played him (and generally I think everybody at the table, including the GM, especially the GM, absolutely love the PC and how I've worked him) it's just too much of a temptation to get all weird and wicked in a game.
So, yeah, I don't go in for ANY of that in my games. No matter how well done, no matter how poignant and full of good RP flavor, stuff like PK and evil just get their fingers into the game and start to mess it up. I probably could have had just as much fun playing a paladin, say, and we'd have never had to sit back and wrench the game back into alignment in the middle.
--fje