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Derren Brown produced an excellent show about this kind of phenomena. He reasoned that people lose their decency and inhibitions and act like morons when they can act anonymously in a crowd.
This is why perfectly nice, ordinary people troll online and why normal people can get involved in football violence.
He set up a fake gameshow, and everyone in the audience had to wear a mask over their face. They made decisions that affected a guy sitting in a nightclub who didn't know he was surrounded by actors. For example, they could either vote for a hot girl to flirt with him, or a muscly guy to accuse him of looking at his girlfriend and start a fight with him. It ended with the entire crowd shouting and cheering as someone broke into the guy's house and smashed his stuff with a baseball bat.
It was called "Derren Brown: The Experiments - The Gameshow". If you're in the UK, it's on 4OD. If you're outside the UK, I highly recommend you try and get hold of this show and watch it.