Inexpressive, not very good actors make terrific sociopaths
(By the way, mediocre or not good doesn't necessarily mean horribly bad)
- Richard Gere in Internal Affairs
- Alec Baldwin in Miami Blues, The Juror or Mercury Rising
- Michael Madsen in Reservoir Dogs, Kill Me Again or the Getaway remake
- Tom Cruise in Collateral
All of them found their moment of glory portraying psychopathic characters.