Very, very few people actually study serial killers and psychopaths. Most psychologists working with offenders will study the "bog-standard" ones who have committed far lesser offences. Criminal profilers mostly work within the police force. You're best off studying Forensic Psychology, getting some work experience with offenders in prison once you're old enough and/or with victims of crime, and then doing a Masters in it. The Masters I'm starting this year has specific modules on Offender Management etc.