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.

Humans, Domination Behavior, And Denial

As a function of self-preservation, Human Beings tend to adamantly deny traits about themselves that could be seen as flaws, or that expose their 'weaknesses'. We deny things about ourselves, but we also deny things about our entire species Homo Sapien. Accepting that the species itself has inherent flaws or weaknesses would mean that each of us probably have them too, so we tend to deny all of it categorically. 

One of the "flaws" that we as a species tend to deny about ourselves with great passion is that Humans have inherent Domination issues, along with a desire to create and live inside of hierarchies.

We have a hard time getting along with one another due to these very issues, whether we're talking about people in our home, at work, in school, in the community, or other countries.
The problem is not ONLY that we are born with the tendency to dominate and control other individuals, but also that we keep denying that we were.

The evidence is all around us, all day, every day, everywhere we look.
Humans have Domination compulsions.
Humans get a huge kick out of competing and winning.
Humans have negative emotional reactions when they DON'T WIN something; they call it "failure".
Humans are so obsessed with "winning" that they create actual awards and hold huge, ongoing contests and competitions, and think nothing of paying money to watch or participate in these contests.
It's considered a HUGE RECOGNITION to be the Winner of an Award, a competition, a contest, or a game, and the bigger the audience, the bigger the "importance" of the WIN.
Humans tend to desire higher status, superiority, and control over other humans.
Humans tend to look for "reasons" to be recognized as having more authority than others, and being considered "more worthy", "more capable", and "more valuable" than others.
Humans tend to look for "reasons" to judge another person as LESSER than themselves.

Humans get a "charge" out of controlling and dominating other human beings, and being SEEN AS A PERSON WHO "WINS", (who "dominates").

Humans tend to give very, very large amounts of "respect" (and also resources) to those who have WON something, regardless of anything else about the person.
They will often freely give Leadership positions to those who have received some kind of documented Award or Degree, regardless of anything else about the person, and regardless of actual experience or knowledge in anything else.   

Humans tend to automatically treat those who are larger or appear stronger than they are MUCH DIFFERENTLY than they treat other Humans who are smaller or appear less physically strong than themselves.
In fact, Humans tend to go to extremes with the way they assess other Humans by size, height, and appearance, judging each and every person they see for "status" in comparison to themselves, and for "status" in the larger group.

Humans do NOT assess and judge others by the person's actual behaviors, intelligence, ability, or the way the person treats others.
Obviously some do these superficial assessments more, and some do them less. It depends on the level of awareness and self-awareness that a person has, and their ability to think critically and abstractly, along with their capability for being objective (detached from their emotional reactions and ego issues).

The Human assessment and judgment of others is first about whether another person is a physical threat (think tribes, clans, and roving marauders of earlier times). Immediately after that, it's about placing where another person stands in comparison to themselves in "status" (again, think hierarchy in tribes, clans, and kingdoms), and how the rest of the group sees and treats the person. It's done mostly unconsciously, without much awareness, usually with no actual logic or conscious thought, and very quickly:

~"Is this person physically larger than me, or smaller?"

~"Is this person stronger than me, weaker, or similar in strength?"

~"How does the rest of the group see this person, do they think he or she is important?"

~"Is this person more 'important' than me, or less 'important' than me according to the group?"

~"Is this person the same sex as I am, or the opposite sex?"

~"How do others treat members of that person's sex, do they give them automatic higher status, or automatic lower status?"

~"What is this person's ancestry, and is it the same or different than my own?"

~"What is the accepted, common STEREOTYPE that matches this person's appearance, and are others treating the person that way? Can I get away with treating the person that way?"

~"How does this other person's size compare to the rest of the group? How does my own size compare to others in the group?"

~"Can I personally get away with treating this person as lower status?"

~"Would others join me in treating this person as lower status, or would they disapprove and give me consequences?"

~"Do others like this person or dislike this person?"

~"Do others listen to this person, ignore this person, or bully this person?"

~"Do others follow this person's lead, or not?"

~"What is this person's sexual orientation, and what status does that give them in the group?"

~"Do others seem to fear this person's reactions? Should I fear this person's reactions?"

~"Does it appear that I could physically overtake this person?"

~"Will there be consequences, and what would they be, if I were to try to dominate this person?"

~"Can I assert control over this person without getting seen or caught by others?"

~"Will anyone stand up for this person against my disrespect toward them, or against my domination or control over them?"

~"Does this person seem to have status in the group or community?"

~"Does this person appear to have financial wealth, or some kind of power in the community or group?"

~"Does this person have something I want?"

~"Do I find this person physically attractive?"

~"Do other people find this person physically attractive?"

~"How socially savvy does this person appear to be?"

~"How do others seem to treat this person in comparison to the way they treat me?"

~"Does this person accept me?"

~"Does this group accept this person, or me, as 'One Of Them', or as LESS important, or as MORE important?"

~"Am I being treated with more positive attention than others nearby? How is this other person being treated by the other members of the group?"

This is a very basic short list of questions and assessments that run very quickly through the average person's unconscious mind when they meet or see another person. There are more, many more, and they do vary according to the individual person's motives and mental health.

"CAN I DOMINATE THIS PERSON, OR CAN THEY DOMINATE ME" is the bottom line; all the other assessments are built upon that.

Those who have more Domination compulsions and issues of course will assess and judge others within that context, and tend to assign "status" to others and to themselves regularly in their minds.
They also tend to be reactive when another person does not seem to accept the "status" that they've projected onto them (they get upset when others stand up for themselves, or when others don't seem to take "being important" or "hierarchy" seriously).

Those who have few Domination issues tend to compare themselves and others less; tend to be able to assess others by their actual "person" (the way they treat others and their strengths and abilities), NOT by their appearance; tend NOT to feel threatened by the assets, talent, abilities, or looks of others; and also tend to take their OWN integrity, behaviors, and actions (the way they treat other people) very seriously.
Because of the Domination issues and behaviors of other people, many Humans tend to feel FREE when they "get out from under" by achieving financial wealth or fame. They feel FREE because they feel that they are ON TOP, and therefore can't be so easily dominated or controlled by the other Humans around them. They feel FREE because they have enough resources and "status" to have more control over their own lives, and be much less subject to the control and domination issues of others.

Whether a person has more or fewer Domination compulsions and Status issues appears to be directly related to their self-awareness, their level of objectivity, and their level of actual integrity.