I think the issue goes beyond moral grandstanding. If you read Hacker News, for example, you see plenty of technical grandstanding every day. I think we humans are simply unprepared for interacting with random strangers online. Our monkey brains did not evolve for that. In most social situations, our opinions are tempered by knowing the other person, or at least being in the same room with them. It's rare that you wo…
I think this is the most important part. It's not just online vs offline. Even how people converse in DMs differs from public postings (not as good as offline, but different than public online). Instead, many interactions are done with the intent to gather an audience that agrees with you, so you can beat down your opponent by what is essentially intimidation. Or at least make it fruitless for your opponent to even try to defend themselves.