Grace This is an excellent Question because it's one that is asked quite often. Although the answer is terribly complex, it's important that it be answered clearly, fully & professionally as humanly possible. It is neither acceptable nor helpful to the Mental Health community nor the balance of the population for this to remain a mystery or an unaddressed concern.
Needless to say, any response here in our space provided will be but a simplified explanation that can be expanded upon through research of one's own abilities.
Try to think about the vast number of times, when discussing perhaps a discovery of an atrocious & bloody scene of mass murder. People from all walks of life will more than likely respond not only in horror & shock, but by stating that "only a psychopath (mentally ill individual) could be responsible for such atrocities!") There will be mention of "pure evil," within the ensuing conversations and investigations.
Does the term "psychopath," instantly and always lead to a description of "evil?" The only correct answer to this, Grace, is "Yes" but with distinct exceptions." At this point those exceptions must be clearly identified & carefully explained.
Any form of pathology has it's degrees as well as general understanding & levels at which society will base their tolerance upon. To be specific, please consider for instance a "compulsive liar" aka a "pathological liar." These individuals although a societal issue and concern, are very rarely responsible for life & death situations. Certainly these people are far from having homicidal tendencies.
Although these are not admired nor respected behaviors, few people would equate this with a term as harsh as "evil."
At the very furthest end of the spectrum, take the infamous Ted Bundy or Jeffry Dahmer, known serial murderers of the most extreme nature. We may ask: Are these individuals crazy~sick~pure evil~psychotic~?? what? How exactly do we as stable, rational human beings, come to terms with such human flaws? Were they BORN with an "evil gene?" OR, were they so deeply & irreversibly traumatized/injured in their youth that a severe mental illness developed and was exacerbated by abuse & neglect?
I hope this will encourage you to see the enormous picture in these cases that must be examined, measured, compared & understood in order to keep the line in focus between "evil" and psychopathic behavior.