I put up a queation along these lines some months ago but what I am planning now is a step further. (very slight Shattered hope spoiler)
I ran shattered hope as my first scenario and one of the players (the adept) ended up rescuing the stone. Since then he has recieved aid several times from the stone (and been punished as well). Although there has been suspicion about the player it has not really developed. However, at the end of their last mission he recieved a critical wound (lost a leg) and the party's cleric was present twhen the stone healed him. Although the player tried to convince the cleric that the stone was a useful tool that had helped him several times, the cleric decided to purge him (quite rightly). At this point I stepped in and the daemon trapped within the stone stopped the cleric (gave him a nice permanent wound -3T and a hatred of demons). The adept (now an NPC) fled the station and took control of the ship the party arrived on leaving the rest of the team marooned on an abandoned space station.
I have been running for the campaign for about 9 months and am really enjoying it. However, my wife is pregnant and due in October so I will need to take a break from my campaign about then. Therefore I plan on leaving it on a quite dramatic point, maybe a cliffhanger. One of the things I am considering is the adept killing one of the other players in cold blood execution style in front of the rest of the group. Maybe as part of a ritual.
The question is, Is it unfair to decide to kill a player for the benefit of the campaign?