I think one of the reasons the Internet is so hostile is b/c people often use memes that’s sole purpose is to ridicule or mock the opponent. These often include pictures that ‘trigger.’ These don’t contribute anything other than to rile the opponent up. A picture’s worth a thousand words. You can’t just flash an ‘offensive’ picture in flesh-and-blood conversation.
Several reasons it is mean.
1)anonymity. In some cases nobody knows it is you but the forum itself if you are required to give an identity and can't figure out how to give a false one. People that think that they won't get caught for doing things, tend to be more apt to do them, our human nature to sin.
2)no direct contact. When you are in public people can get in your face where you are the physical presence can be intimidating and often there are peers around that can discourage you from wrong deeds. Online if nobody is spying on you, the only ones stopping you from acting out is the places you are on social media.
3)group mentality. When everyone around you is screaming things at someone and few are standing up to it then you either want to get out of there, be quiet, or are swarmed by those disagreeing with you.
4)moderation that encourages certain types of bad behavior and discourages countering behavior to it. People see that they can attack others and when they likewise attack back they are censored or banned which more than renders their attack ineffective. One could say that certain sites are safe havens for bad behavior (meanness).
5)Ungodliness abounds. God is nowhere anywhere online to speak of and when people do counter arguments with God as a standard they are attacked, censored, and banned into oblivion.
When the majority of people hate God then hate is what their civilization devolves to. We don't hear of any other "good" nation in history that hated God. Those who deny God a honorable place in society at least have a sense of honor, those who ban him, ban honor and without honor people don't respect one another and being mean without good cause is what happens.