Inspired by Pittplayer's challenge, I've been wracking my brains trying to work out how to make 'traitors' work. Giving them bonuses whihc help them achieve 'Join the Winning Team' is all very well, but it's not very imaginative and to be honest, it's a bit dull because the other investigators can't actively do very much to 'stop' the traitor. In fact this is the main problem. There are quite a few items which will help a traitor make the game more difficult, but there aren't many counter-measures for right-thinking investigators to use. So it becomes a mere race rather than an actual battle, and the two sides aren't even racing for the same goal.
Anyway, I went through the item decks and tried to identify which items help screw over the other investigators. There aren't many - maybe a dozen that really work. Here we go:
Join the Winning Team - too obvious. There's no need for a 'traitor' because this makes any investigator into a traitor - just not usually a very successful one.
Call Ancient One - too dangerous. If the 'traitor' character has any incentive to awaken the AO, this card makes it a bit too easy and will spoil the game.
Call Friend - ironically this can be used as an absolute pain. If a traitor has it, he can use it to pull investigators off gates, stopping them closing.
Summon Monster - if the traitor has the ability to evade the monsters he creates, he can run around spawning monsters all over town.
Lure Monster - same principle as 'Summon Monster' applies.
Implant Suggestion - this is quite good fun for the traitor, but it's hard to make a big difference with it.
Arcane Insight - this can be used to make this immeasurably worse as well as immeasurably better.
Massa di Requiem per Shuggay - same problem as 'Call Ancient One'. Either you incentivize it (in which case it's too good) or you don't (in which case it's no more useful to a traitor than to anyone else)
Mi-Go Brain Case - you can cause havoc with this if you do it right, because the other investigator doesn't have to consent to the swap. I have a question though: when Brain Case says "spend all your movement points for the turn", can you first use extra movement points from a motorbike? If so, you can pull off all sorts of stupid stunts using this item - move on to a whole heap of nasty monsters using motorbike movement points, then swap with someone else who has already moved that turn.
Naacal Key - potentially lethal, but you can only use it once. Opening a Rlyeh gate at the Asylum is probably the way to go.
Milk of Shub Niggurath - the ultimate superweapon of monster control, but it can't be used more than once.
Astral Mirror - in theory you can use this to get rid of a beneficial environment card - but in many cases environment cards simply apply low-level annoyances to all investigators, and traitors might be just as inhibited by those as anyone else.
Gate Box - can be used to get other people Lost in Time and Space, although it requires them to be foolish enough to enter a gate when they know you're about to come out of one. Of course they can't just ignore gates for the whole game, but it's not likely you'd get this to work many times.
Plague of Locusts - this spell is (I think) the only one which will actually directly hurt other investigators. You don't need to exhaust it, but it's very self-destructive so it will rarely be worth it.
Premonition - I just noticed that it might be possible to use this to move someone else's skill slider, although that's a bit of a generous interpretation.
Special mention also goes to the Book of Anubis exhibit item, which heals you when bad things happen to other investigators.
Looking at that lot, I think there's potential for a Mi-Go character (who has some special ability which helps him abduct people using the Brain Case) and a sort of evil version of the Dreamer, who has an ability which helps him use the gate box to get ahead of people in OW and take the gate away. But none of these ideas solve the real problem of how the other investigators can fight back.
Any thoughts?