A sociopath is a person with a mental illness that manifests in extremely antisocial behavior and the inability to distinguish right from wrong. You're probably right to be afraid of a sociopath.

Pathology means illness, and so socio- pathology means social illness. If someone is a sociopath, that means that they have a social sickness––they lack the ability to be social. Many movies and TV crime shows feature plots involving sociopaths, but hopefully the TV is the only place you ever encounter sociopaths.

Definitions of sociopath

n someone with a sociopathic personality; a person with an antisocial personality disorder (`psychopath' was once widely used but has now been superseded by `sociopath')

Type of:
mental case, neurotic, psychoneurotic
a person suffering from neurosis