I think villains are anyone who acts in opposition to all that is good and represents the worse aspects of humanity. It's only recently in the 21st century that the term "heroes" and "villains" have been relegated to comic books, and movies, but the concept of the hero (order, the ideal good) and the villain (choas, all that oppose humanity) have been as old as civilization.
The first proto-villains humans mentally conjured up were symbolic of things in nature that presented a threat to human survival; from the animal kingdom, giant beasts, serpents, and dragons, from the nighttime, shadows, and ghouls, and from the weather, deities, divine entities, and spirits. Those threats were eventually inbedded into our cultures as myths, legends, and religions, with those who stood against the forces of choas (think Horus, Odysseus, Christ) placed as Gods and heroes, and those who embodied choas and behaviors that were either detrimental to humanity akin to the beasts of nature where placed as villains (think "Serpent in Eden" Satan).
Humans who embodied attributes of those evil archetypal figures of legend were also titled villains. Vlad the Impaler, Ivan the Terrible, Pol Pot, Hitler and Stalin, etc. I think because of how hero and villain archetypes have become relegated to media entertainment in the form of comic books, and movies and manga, rather than great epic stories that embody the human condition is why the term villain has become colloquialized.
For example, when someone calls Trump a villain, it is because to them, he embodies the archetype of the tyrant, but when most people envision a tyrant, they see Skeletor, or Darth Vader -- Thanos, or Freiza. They don't think of real tyrants like Ivan the Terrible, otherwise they wouldnt be relegating the title of villain to a U.S. president. Even an honest comparison of Trump to Hitler is embarrassingly naive. I dont think villain needs to be defined or redefined, I just think society is in a really bad place right now.
EDIT:
After dwelling on it for a while, I think I know what the issue is. The archetype of the villain of the past sought to bring choas to order and tyranny (organized evil) to freedom, whereas the hero of the past sought to bring order to choas and liberate those under tyranny. I think that dichotomy has been reduced to liberty vs tyranny. In today's society, heroes are only seen as liberators whereas anyone that attempts to bring order is tyrannical, regardless of whether they're a hero or villain.
Ex. People identify with the aspects of Christ that symbolize liberty in the form of redemption (freedom from the burden of sin), but see aspects of him that symbolize order in the form of reenforcing the laws of God as tyrannical.
Here's a better example; Pope Francis embodies liberty with his stances on homosexuality, migration, etc., while ignoring his duties of bringing order to choas, which is why he's easily the most idolized and popular pope in recent times. If he embodied the archetypal hero of old who brought both liberation and order, he would be seen as a tyrant.
