The following is the story of Parn, the most broken character I never wish
to meet, and the only character I know of who backstabbed two parties
worth of characters in one combat.  Look on this history, ye player
characters, and learn from our mistakes... 

====

I joined [the D&D group] Ragnarok in a campaign run by a guy named Ben. 
Ben's fondness for handing out major magic items to low-level characters
was matched only by his tendency to kill those characters with those
items.  (At one point our fourth- and fifth-level party ran into the
Shadowstaff, a Deck of Many Things, and an ego-34 chaotic evil sword, all
over the course of two combats.  We died a lot.)  For all that, I still
thought he was a good DM.  Our party made some silly choices at one point
in the game, and I think we had badly diverged from his prepared
storyline. 

Shortly after winter break, Ben quit Ragnarok, and another DM took over. 
Out of an original party of six, two players were still alive.  Those
players had ridiculous amounts of stuff; the rest of us had new characters
with no equipment.  One of those original characters (Mitsumeru by name) 
chose that point to leave our party and become an NPC.  She took about
half our stuff with her, including an artifact that we had to have to
finish the quest.  It was in the battle to reclaim that artifact that most
of our party eventually died. 

The last of the original characters was Parn.

...A few sessions later, our party met up with another party controlled by
a different DM.  We joined forces for an assault on a powerful NPC. 
Things were looking good for a bit...  Then a new player, Nasri by name,
joined the party with his evil wizard character, over my loud
objections. 

Nasri's character took Parn aside (away from the main party) and led him
through an evil ritual involving multiple human sacrifice, the purpose of
which was to allow him to become a green slaad a few days later.  What is
a slaad, you might ask?  All that's important, really, is that it's big.
And bad.

Our party didn't know exactly what had happened, of course.  We did know
that the evil wizard and the chaotic-neutral fighter had vanished for a
few days with a bunch of slaves they had freed, and that when the fighter
returned, he was acting awfully paranoid and his lawful-good sword was
refusing to heal him.  

I tried to organize some sort of action against him (involving capturing
the sword and asking it what had happened), but met with resistance from
the rest of the party.  "If we find out what he's doing", one person said,
"he'll probably attack us, and then we'll have to kill him.  And it would
really be mean of us to kill his character after he's been through so
much..."  I kid you not.

Well, our party split apart again and went our separate ways.  One party
contained those players who were willing to tolerate Parn in their group;
the other contained those who did not.  Parn became a slaad, and the rest
of his party shifted from being "companions" to being "tagalongs".  They
did get a bunch of experience, though...

During the last session of the spring semester, the parties rejoined and
went after Mitsumeru, who was holding one of the last artifacts we needed
to obtain.  We had a total of ten characters in the combined party at that
point, but Parn as a slaad was still powerful enough to do whatever he
wanted.  We received a graphic demonstration of this when, in the middle
of the combat with Mitsumeru, he backstabbed the entire party.  Three PCs
escaped that battle; most of the rest were killed while running away.  As
an afterthought, Parn finished off Mitsumeru and collected the rest of the
quest artifacts the rest of the party had dropped.  And there it ended.

====

For a while I was drawing some sort of moral from this adventure,
something to the effect of "do not allow evil characters in your party."
Later I had cause to reconsider, though.  If you ban evil characters,
players will simply declare their alignments to be neutral, and make sure
to provide themselves with motives to act evil where appropriate.  The
secret is to ban evil PLAYERS from the party...

Actually, I think the mistake I made here was one of roleplaying.  
 A literal definition of "roleplaying" is "a policy of doing what one's 
 character thinks is best, rather than what might be most interesting for 
 the player".  Many people declare that their characters are bigots, 
 paranoiacs, morons, et cetera, and do obviously stupid things with the 
 excuse that "my character doesn't know any better". 

 I believe in playing optimally in any game I'm in, so I try to avoid that 
 sort of thing.  One aspect of roleplaying still applies to my play, 
 though.  If, from my character's perspective, the best thing to do is 
 leave the party, than my character must leave the party even if this 
 would result in me leaving the game.  I have applied this rule in 
 subsequent adventures, and I wouldn't say it's worked well, exactly, but 
 nobody has backstabbed me since...