PG: The biggest source of stress for me at YC was running HN

People were always like this. You just weren't able to see it, because you were in an offline "filter bubble" of peaceful and reasonable people. There have been countless civil wars in history after all.

No, what online media are doing is massive propaganda at scales and levels of insidiousness unlike ever imaginable before now. Cambridge analytica for instance targeted ads at people to maximize their outrage toward other citizens. Before advertisements were much easier to avoid or ignore and weren’t being used for outrage. Ads have to get clicks nowadays for revenue and in the past they just had to be associated with wide viewership. People click on what infuriates them more often than other content.

Long before the internet there were people generating enough outrage in a society to slaughter millions of people on flimsy or no basis. I don’t think this is anything new.

If anything, it offers a counterbalance: you can also communicate with groups of reasonable people not physically proximate to you.

Not in the postwar West, and that's the area where the change we're talking about is notable, so your point is actually an argument for the opposite conclusion.