noun
Derogatory term for a person with abnormal sexual tendencies or morally corrupt behavior. Often used in news reports about sex offenders.
警察は変質者を逮捕した。
The police arrested a pervert.
夜道で変質者に
Be careful not to encounter a degenerate on the street at night.
変態 is a more common and casual term for 'pervert', often used in everyday speech, while 変質者 sounds more formal and is frequently used in legal or news contexts.
Compound of 変質 (henshitsu, 'change in quality; degeneration') + 者 (sha, 'person'). The word literally means 'a person who has degenerated', implying moral or psychological corruption.