There are many pervasive, interconnected issues and trends to it, so there's no simple or complete answer:
0. Behavior online doesn't arrive out of a vacuum. If people are miserable because they're working harder, making less and their society is in retreat, they're probably going to take out their frustrations out on the easiest targets.
1. Unplug from social and mainstream media because it doesn't have much value.
2. Stop demonizing, hating on groups of people and falling for the unthinking of mob tribalism, even rhetorically. Hate bad ideas, not people.
3. Realize that we're divided-and-conquered if we're going to let a few rich people and their corporate media keep us set against to each other. Solidarity is the only way.
4. Anonymity is good in small doses, but it's too easy for people to act unreasonably hiding behind it (cyberdisinhibitionism). DHH's company wrote a blog article about improving the quality of discussions with profile pictures and real names.
5. Stop and use every instance of it as a teachable moment, where feasible. It only works though if people have shame and can be brought around to the Golden Rule/empathy... it seems to me most parents these days aren't as involved in active parenting, so their kids run ferrel and so more people grow up to act more brutally and sociopathic. Furthermore, the current prevalence of parasitic vulture capitalism valuing myopic greed and selfishness above all else reinforces a disinterest in the concerns and well-being of others... which is antithetical to community and civilization.
6. There's nothing yet so far to replace the community function filled by religion, and so many people aren't interested in behaving themselves or doing right by their neighbors or strangers if they can get away with it.
7. More people have lost most of their hope about the future. For example, no healthy society has mass shootings/suicides nearly every day that no longer make the news.