What did a mercenary do during the renaissance? why does the term mercenary carry such negative associations, or connotations?

Mercenaries are soldiers being payed who does their work with their own personal gain at mind. They focus more on their agenda rather being loyal to their employees. These mercenaries were skilled freelance fighters whose loyalty are among themselves and sold their abilities according to the highest bidder. In this sense, mercenaries carried a negative connotation for being cunning and witty in selling their abilities. They could perfidy their employees when they don’t see anything good to stay with them.  

D.