Here are my thoughts about the immorality of "income inequality"...
The causes of "income inequality" are actually very simple:
1. The malicious/devious cunning and greed of the aggressor (who eventually becomes "rich/upper class"), and
2. The cowardice, stupidity, and/or greed of the victim (who eventually becomes "poor/lower/middle class").
Before money (in whatever form) can even begin to exist, i.e. to become legal tender, a certain number of people must co-operate and accept it as a currency.
For example, if a certain man somehow manages to own a certain number of, say, gold bars, he would neither be rich nor poor, as long as NOBODY values his gold.
Therefore, this man (the "aggressor"), in order to become rich, needs to somehow brainwash, bribe and/or bully (the 3 "B"s) a certain number of people (the "victims") into THINKING that having a CURRENCY is a GOOD thing.
So that's how money/currency, and consequently, "income inequality", begins to exist, especially on a worldwide scale.
It needs the worldwide CO-OPERATION of both the rich/upper classes (motivated by malicious/devious cunning and greed) AND the poor/lower/middle classes (motivated by cowardice, greed and/or stupidity), simply because if ALL the poor and lower/middle classes were to be brave enough go on some kind of lifelong strike from money-paying work, ALL the monetary wealth of the rich and upper classes would become WORTHLESS as it would NOT be able to BUY the SLAVERY of ANYBODY (which used to be the poor and lower/middle classes) to produce products and provide services for the rich and upper classes.