So we recently had a player get this insanity where he wins if the other investigators lose. It resulted in him purposely killing himself forcing the scenario to end. I looked up the errata and it seems they have added to the base rules where if an effect doesn't specifically say you will lose you don't win so in our case, everyone lost including the insane player. The issue I have is that the other players have no real agency to stop an insane player from causing issues. So even if he can't "win" by suicide, he can force a scenario to fail without any way of stopping it if he has been outed as a traitor so that no one wins. I like the idea of having this card because it means any insane player could potentially be working against the team. However the fact that you really can't do much outside of not letting them do puzzles or important NPC conversations means they have a lot more power to screw with stuff and can't be really stopped if they don't want to be sneaky.
I think a good house rule to this card is that once you get it, if you are ever defeated you reveal your condition and you do not count as a defeated investigator. This works thematically since once someone is a traitor why does the group care about them? Game play wise it means the other players can leave him or her to die or whatever if they KNOW that player has gone bad but it also means that with that option they can potentially let a good person killed (who they thought was fishy and couldn't trust) which would mean they would lose leading to more trust/distrust dynamics. Has anyone else had issues with this sort of thing?