A hireling is someone doing a job solely for the money. The work they do is therefore tainted. A hit man might be dismissed as a hireling by the FBI, who wants to go after the mafia boss who is hiring the murderer.
If you have been raised with the idea that there is dignity in hard work of any kind, it may be hard to understand the negative connotations associated with the word hireling. Like mercenary, it implies that some jobs should be done only out of moral conviction––a hireling will never have the right attitude or love to do things right.