No, mercenaries do not earn money for killing, any more than simple murder is the job of a soldier. They have missions and objectives, which with mercenaries would usually be to protect things but may involve being part of a larger regular army unit. At least that is the case with official mercenaries. Bad men in war torn countries do all sorts of things.
In generally I think the main thing you gain from killing someone is a place to live for a while (prison).