Sentence:  Landesco thought him `` just a superior sort of plugugly '' but he was , in fact , with his aggression and hostility , and nerveless indifference to risking or administering pain , a casebook <WSD>psychopath</WSD> . 
Meaning of the word marked with <WSD> tags:  someone with a sociopathic personality; a person with an antisocial personality disorder (`psychopath' was once widely used but has now been superseded by `sociopath')