Some people are mean online because of anonymity and because they don't use their real names. This is a popular explanation for why people say terrible things and make terrible claims on the internet but even when we account for internet trolls and anonymity the vitriol of an internet troll and the anger of someone who joins a lynch mob may not be that different in terms of sociological or emotional dynamics. If you combine the anonymity of the internet with a diffusion of responsibility and a setting in which people don't conduct themselves as they would if they knew people what you see on the internet is probably not that different from herd behavior in other settings.
So, yes, we're all capable of being that mean but many of us get embarrassed being that mean in more direct and personal relationships.
Some have camped on the observation that we're meaner in anonymous and on-line settings than in real life. Well, maybe, but some people are pretty much the same in an on-line or public persona as they are in face-to-face contact.
In ancient Roman society there was the spectacle of gladitorial combat for amusement. Now the gladitorial combat may not involve bloodletting but things like MMA and online battles may play the same sort of role. Some people get a visceral thrill watching people physically fight, others get that visceral thrill in more abstract gladitorial combat.