The complete text of the classic book on psychopaths and the psychopathic personality The Mask of Sanity is available online as a pdf file.
The book was written by psychiatrist Hervey Cleckley (pictured on the left) and is one of the classics in the field. It is still highly regarded for its in-depth case studies of psychopaths.
Contrary to most Hollywood depictions psychopaths are not necessarilly people who enjoy causing pain or suffering, but are thought to lack empathy and, therefore, tend to use violence to acheive an end, without concern about the impact of their actions.
There’s further information on psychopaths and psychopathy here for those wanting an introduction.
Link to complete text of The Mask of Sanity (1.4Mb).
Link to webpage on psychopathy.