There are several synonyms for the term "professional murderers." One of them is "hitman" or "hired killer." Another synonym could be "assassin," which typically implies a political motivation or connection. A "contract killer" is someone who is hired to commit a murder for a specified fee, while a "gunman" is someone who uses a firearm to carry out a killing. The term "serial killer" is also often used to describe someone who commits multiple murders over time with a specific motive or pattern. Regardless of the term used, all of these synonyms describe someone who takes lives for a living, whether it be for personal gain, political reasons, or sociopathic tendencies.