Derogatory term for a person with abnormal sexual tendencies or morally corrupt behavior. Often used in news reports about sex offenders.
The police arrested a pervert.
Be careful not to encounter a degenerate on the street at night.
変人 just means an eccentric or quirky person, but 変質者 implies someone with a disturbing or perverse mental abnormality, often criminal in nature.
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.