noun
abnormal person; deviant; pervert; weirdo
A person whose behavior or thinking is considered abnormal or deviant. Can be used in clinical, judgmental, or colloquial contexts. Often carries a negative or stigmatizing tone.
彼は周囲から異常者扱いされている。
He is treated as an abnormal person by those around him.
あんな行動をするなんて、異常者としか思えない。
Acting like that, I can only think of him as a weirdo.
異常 is the abstract noun/adjective meaning 'abnormality' or 'abnormal', whereas 異常者 specifically refers to a person.
Compound of 異常 (abnormal) + 者 (person).