How people manage to be extraordinarily rude on internet?

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Face-to-face, being rude is not so easy. Speaking rudely while looking at a person’s eyes is easier said than done. But on internet, people become very brave. There are certain people who speak very rudely on the internet. This often happens while chatting. I have heard of many cases where some people have been called vile names. Is it just because people cannot see each other’s face? Or do you feel there is some other reason? What, in your opinion, could be the reason behind people’s rude behavior on the internet?