noun
mass murderer
Refers to a person who has killed many people, often in a single event or over time. Neutral, factual term.
その大量殺人者は終身刑を言い渡された。
The mass murderer was sentenced to life imprisonment.
警察は大量殺人者の身元を特定した。
The police identified the mass murderer.
Compound of 大量 (large quantity) + 殺人 (murder) + 者 (person).