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.
person with a personality disorder; person with an abnormal character
mentally ill person; person with a mental disorder
Compound of 異常 (abnormal) + 者 (person).