A more thorough explanation:

A mactator is a person who has committed murder. The word comes from the Latin word "mactare" which means "to slaughter".

John was convicted of being a mactator after he killed his neighbor in a fit of rage.

The police are searching for the mactator who killed the victim in the park last night.

The examples illustrate the definition of mactator as a murderer. In the first example, John is referred to as a mactator because he committed murder. In the second example, the police are searching for the person who committed murder, and they refer to that person as a mactator.