Refers to a person who commits murder; a formal or legal term. More common in news or official contexts than in everyday conversation, where 殺人犯 is often used.
The police identified the murderer.
The killer fled the scene.
Compound of 殺害 (satsugai, 'murder, killing') + 者 (sha, 'person'). The exact historical derivation is uncertain, but the word follows a standard Sino-Japanese pattern for agent nouns.