Why is low self esteem a reason some people shame others?

The above answer is good one. A person usually seeks to
shame another to make himself or herself feel better. By bringing a person down, a
person with self-esteem issues feels better in a twisted way. That said, I will add a
social dimension.

Why does a person have low self-esteem?
Usually a person has low self-esteem, because he or she does not have certain qualities
that he or she deems society values. So, if a society prizes wealth or beauty and a
person feels as though he or she lack in this department, then this person will have low
self-esteem.

If we build on this premise, we can say that
shame also has a social function as well. For shame to work, person must feel the
displeasure of society. So, a person with self-esteem issues may seek to bring another
person down not to to make himself or herself feel better, but to have another person
who is outside of society. In short, misery loves company.