In my last campaign, the players were good and fighting various forces of varying degrees of evil. One day we were going to be short a player (I do not run if one of the players can't make it, but there are only 4 so its a small and very regular group).
So, what I did was made up 2 Half Orc (brothers) and a Duergar. This game would have them running a mission (quest) for the other side (their character's enemies) to take out a tower said to contain some magic that would help defend the realm from these forces. The players themselves knew this was a valley their characters were to defend. My players were mature enough that they put in 110% even though they understood it would make their regular characters job that much harder.
It was a great way to be involved in the campaign, despite missing a player, and to give them something different that they had not done before. The tower as it was, was under water (only the top was visible). They knew nothing of it other than it housed some kind of magical defense - but it was probably well guarded and so these three represented elite members of that side who were to infiltrate and destroy/capture the place.
Long story short, let them play some bad guys with a specific mission or quest against the 'good guys'
It's an idea, hehe.