Someone who's sociopathic is mentally ill and unable to tell right from wrong. Sociopathic people don't feel guilty about things they've done, because they lack a conscience.
The word psychopathic came first, from the Greek roots psykhe, "mind," and -path, "disease" or "suffering." You'll see path in many medical terms, including neuropathy, or "nerve disease," and pathology, "the science or study of disease." Sociopathic was coined in the mid-twentieth century, with socio- referring to the inability of sociopathic people to interact with others in a normal social way.