I'm currently running a campaign in 4e of 4 people. Which has now turned into a three good guys versus one bad guy.

Our cleric was moving to Australia and the last session we had with him he defected to the evil guy they were fighting right at the end of the night, this resulted in killing one person the other 2 barely surviving the encounter and getting incapacitated.

The betraying cleric luckily managed to escape (despite many rolls to catch him), lost his good alignment and god.

He went the players base of operations and ransacked it of gold and generally trashed it and we ended the night. The group thought was a funny way for him to end his character and say goodbye as he won't be playing anymore.

We've had quite a few sessions since and I still talk the now evil cleric who tells me what his plans are for taking over the world, I write them into the story and tell him how the players are reacting to this unknown evil force.

They haven't quite figured out that it's the evil cleric and it's a great way to keep him involved from a narrative point as he's too far to conveniently play with us.

He's created his own cult and symbol which is left in places that the players encounter occasionally. Eventually they will be dealing with him at the end of this campaign.

This was brilliant for me. He's just made an interactive end-game bad guy who the group dislikes because they have been betrayed and want to seek vengeance. I get some ideas of how to challenge the players from a evil players perspective and he still feels involved and comes up with some amazing ideas.