It is often remarked that capitalism is a beneficial regime because of the "trickle down effect" (https://en.wikipedia.org/wiki/Trickle-down_economics).
>So if society is comprised of 10 people, and you devour one of them and redistribute to the other 9, thats OK simply because the 9 recipients benefit? //
That doesn't benefit all of society now does it?
>If you can gang up and rob your own rich, why not arm up and attack a neighboring country and enslave its population? //
Attacking a neighbouring country won't benefit society it will be a substantial detriment as would enslaving members of society.
The thing about "rob[bing] your own rich" is that the many of the richest members of our societies [in the UK] are rich because of chance, because of rent seeking, because despite the world belonging to mankind they by violence [usually of their ancestors] have acquired a larger share of it's resources. Why is it unjust to redistribute such wealth?
Let's flip your first question - if society is composed of 10 people and one of them has enough food to feed 10, should we just let 9 starve so that the 1 can keep "his" portion? Or to look at it a different way: how about we look at an island that's got resources to feed them all, they each are provided a random share and one has the most fertile and productive part - they all work equally hard and he always has excess and grows fat and rich in resources whilst the others suffer to varying degrees. Why should the benefit born of random chance overrule our humanity?
"We find that increasing the income share of the poor and the middle class actually increases growth while a rising income share of the top 20 percent results in lower growth—that is, when the rich get richer, benefits do not trickle down." (Dabla-Norris et al., http://www.imf.org/external/pubs/cat/longres.aspx?sk=42986.0)
[FWIW I don't agree that GDP is the best measure of improvements in society but it at least appears to indicate the efficiency - within humanitarian bounds - of a capitalist system. I'd prefer to follow happiness quotients.]