why is inequality [overpay-underpay, injustice, legal theft] bad for the economy?

Question

is it bc justice is the glue of society, injustice the cracks and crumbling of society?
is it bc injustice produces violence [war & crime] which are costly waste - cost of judges, lawyers, courts, police, army, damages to property and person, etc, etc]
extreme richpoor middle-east spends around 50% of income on [mostly internal] conflict
egalitarian scandinavia spends 2%
[globally, most army action is internal]
japan, 50% more equal than usa [after gen macarthur's land reforms] eating the usa economy today, despite 87 large cities being firebombed - [eg, 7 of top 10 banks hav moved to japan from usa] ditto germany, 36% more equal - plus: usa forced to pay for japanese german defence as advance bulwark against russia and third world that usa plundered [eg, slaves] - plunderers get poor from their endless defense costs - empires are plundered by those they plundered - the rich get soft, the poor get tough - why usa, russia lost in vietnam, afghanistan - why sicilians conquered usa

Answer 1

Inequalities listed in your question are overpay-underpay-injustice, legal theft.
Overpay results in conspicuous consumption, waste of hard-labour by someone. Just gets thrown away. Results in acrimony of the person who has produced.
Underpay results as a de-motivator and makes a person not to give his best. Loss of produce to the society as a whole.
Injustice brings hatred in mind and the person affected waits for his chance to settle scores, sometimes out of frustration, does anything detrimental to society.
Legal theft- I do not know whether you are referring to the amassing of wealth by a few who are in power and absolute power. here also society as a whole waits to teach a lesson.
Yes, violence can be expected in such a scenario. Inequalities lead to desparate actions by the affected. When affected people stand united, no power can win, as shown in your detail of Vietnam etc., Ultimately every single person's responsibility is counted for a society as a whole to be peaceful. But individual desires close the eyes, not to see the reality.
VR

Answer 2

there will always be some inequality, people are not born equal. The economy can work with both a high and a low degree of equality -- compare Brazil (high) with Sri Lanka (low) for example.