Quote Originally Posted by an_do_89
The mercs were used also because they wouldn't betray . They were getting rich. When a king would expect that his men would betray he would hire mercs for safety. An example is the late Bizantyne Empire , the emperor hired mercs The Varangian Guard (norseman mercs very strong and reliable).

Another thing is that mercs as I know in the medieval ages were spared after a battle and released but I think this was in a chivalry battle.
Well, I guess that's "usually" true...except for the case of rebelling Carthaginian mercenaries or mercenaries hired by Epieros that betrayed them and mercenaries that started working for Romans.