It would be unfortunate to end up alone in an elevator with a psychopath. A psychopath is a mentally ill person with violent tendencies.
Psychopath is a psychiatric term that refers to a person suffering from a mental illness involving violent behavior or various kinds of abnormal social interactions. A person who's been diagnosed as a psychopath has a lot of trouble interacting with other people and living a normal life. The noun psychopath is also used casually to mean simply a bizarre or unpredictably violent person. The word comes from the Greek words for "mind" and "suffering": psykhe and pathos.