Psychopathic is a very strong word — it means mentally ill or insane and probably dangerous.
Psychopathic isn't a word anyone should take lightly. It refers to an undiagnosed mental disorder. Murderers and other dangerous people are often psychopathic. If you watch a TV show that features a character who is dangerous or even deadly, he might be psychopathic. This is a medical word, unlike "crazy," which can mean dozens of things. If you know that someone is psychopathic, stay away: that person is mentally ill and could hurt you.