Sociopaths consistently transgress social mores, break laws, and exploit others, but typically, are also charming and convincing liars who consistently deny wrongdoing.
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The difference between Mello and Light is that Mello does not justify his actions and lets people...
But wait, at the beginning of the discussion, was someone saying that either of them is a sociopath? Neither actually fits what clinical criteria there is (which is not in the DSM, and not defined well on the Wikipedia article or by narcissists like the person who runs Sociopath World).
Either might qualify for an Antisocial Personality Disorder diagnosis, though, which has a muuuuuch lower bar. (“Might” meaning “I’m not sure but it sounds more likely,” not “totally probably does.”)