I don't want to turn this into a “yes … but” war (this is, after all, @papa-pepper's blog) but human beings are not uniquely violent.
Ethologists have found that chimpanzees, for example, do "gang up against weaker ones of their own" and do engage in "ethnic cleansing and genocide" (cf. Jane Goodall's decades-long observations).
We humans are too smart and devious for our own good: we are adept at finding ingenious ways to quickly and easily harm large numbers of other humans. Luckily, we also have ways to tone down or channel our evil impulses into productive endeavours that benefit others.