Sometimes you take a job for the money, sometimes you take it for the location, sometimes you take it for the script; there are just a number of reasons, and ultimately what you see is the whole landscape of it. But I can tell you from behind the scenes - that's what it is, as an actor.
In other words, for every 10 enemy you kill you bring on 20 new recruits to their anti-coalition cause then essentially you are working against yourself.