A person who does something that greatly harms others (innocent or not and they haven't harmed that person) purely to benifit themselves or to cause misery which they seem to enjoy seeing. It is someone who does something extreamly damaging for purely selfish reasons and or to cause misery because they seem to enjoy it. At least that's my interpetation, in books and film the bad guy is just an antagonist, morals have nothing to do with it, all they're there to do is impose the protagonist, who could even be the evil guy.