When Is Rejecting and Despising Someone Okay?

Rejecting, ejecting, ostracizing, or "despising" a person for anything less than a very serious transgression is evidence of either immaturity in a young person, or some sort of emotional or mental illness in an adult. (Which type of emotional or mental illness would have to be determined by the person and a medical or psychiatric professional.)

What's a serious transgression when it might be appropriate to treat another human being with such harsh behavior?  (An adult human being, that is; it shouldn't be done at all to a child.)

~Physical violence
~Abuse, assault
~Purposeful vandalism to a person's property
~Abusing, threatening, endangering one's child
~Abusing, threatening, endangering one's partner, friend, or other relative
~Attempted murder, completed murder
~Purposeful Sabotage
~Stealing, robbing
~Holding a person against their will
~Slander (intentionally damaging a person's reputation by purposely spreading damaging rumors, usually fake)
~Purposely causing another person harm
~Abandoning a person who has been injured or is in harm's way
~Neglecting a person one is responsible for giving physical care and protection to (or mental health care)
~Inciting a group to turn on a person
~Inciting a person's child, partner, friend,  or relatives, coworkers or community to turn on a person
~Actual purposeful Betrayal with no remorse or amends
~Involving another person against their will or knowledge in criminal behavior or corruption

Of course there are more specific examples that can be added to this list. What they all have in common is that they are serious and actual "transgressions"; they are not TRIVIAL, nor are they about personality, ego, common conflicts, social status, or social interactions misinterpreted and gone awry.   

There should be a VERY SERIOUS, VERY REAL REASON when a person is subjected to harsh rejection behaviors by other people.

Making up fake "reasons" that didn't really happen, or wildly exaggerating things that did happen in order to justify one's "harsh" treatment toward another person is a behavior exhibited by those with the disorder known as Narcissism.

Those with certain mental or emotional illnesses will jump to these "harsh" behaviors because it gives them a sense of control and power. It's that feeling of power that they're seeking, either because they're reacting to anxiety and trying to make themselves feel safe again, or because they are looking for a "fix" they get from domination.
The person who they're doing it to is just "collateral damage", they used the person to get that feeling again.
An added benefit is when they're surrounded by others like themselves who go along with it, and may even give them encouragement and 'reward' for their bully behavior.