Refers to a person who has committed murder. Slightly formal or legalistic compared to 人殺し.
The police arrested the murderer.
He is known as a cold-blooded killer.
殺人者 is the formal term for the person who commits murder, whereas 人殺し is the same idea expressed in blunt, everyday language, often used as an insult.
Compound of 殺人 (satsujin, 'murder') + 者 (sha, 'person').