How do you define someone as "anti-social"?

Antisocial refers to people who do things that are harmful to other people in society.

Asocial refers to people avoiding social situations.

What @bolwerk said. I would say that it comes down to the introvert/extrovert dichotomy.
Introverts are depleted by extensive social interaction and extroverts are energized.
True anti-social behavior is a pathology not a preference based on temperament.

Anti-social get’s his or her rifle and goes to the roof when things annoy them. A-social goes to the closet.

Technically, I believe “anti-social” would refer to people that are against social discourse. Oppose freedom of speech.

Asocial, to me, would be people indifferent to social interaction.

Somebody who inhales Nitrous while screaming Mommy loves you

Someone who is antisocial tends to have the some of – if not most/all of in the case of those with Antisocial Personality Disorder – the following characteristics/symptoms: no regard for what’s morally right nor wrong, they can lie or be deceitful typically for the purpose to exploit others, noted to be manipulative of others usually for personal gain or simply for personal pleasure, egocentric, displaying a sense of superiority, they usually have recurring troubles with the law, habitually violate the rights of others by the use of intimidation, dishonesty – manipulation, lying, deceit – and misrepresentation, may commit child abuse or neglect, may be hostile, significantly irritable, easily agitated, impulsive, aggressive or violent, universal lack of empathy and remorse for others, they take unnecessary risks or partake in dangerous behaviors, may be in poor or abusive relationships usually dictated by them and prior mentioned symptoms, may partake in irresponsible behavior at work, and a consistent failure to learn from or disregard for the negative consequences of behaviors they partake in.

In many notable cases of individuals with ASPD (Ted Bundy, John Wayne Gacy, Albert DeSalvo), they’re quite outgoing and charming that is until they torture, maim, kill you.

As @bolwerk said, you probably meant asocial.

Michael Myers.

Stay away from me!

Lots of people probably use the word anti-social when they really mean asocial, I am guilty of that myself. Anti-social behavior hurts people in society like murder, rape, planting a bomb. Asocial behavior is avoiding meeting new people. I more often hear people describe themselves as anti-social when they really mean asocial. Rarely, do I hear people describe themselves as asocial.