If that happens enough, the best users—who don't want to read snark, aggression, petty spats, etc.—will leave, ceding the field to the commenters who do, eventually driving everyone else away and leaving a scorched-earth wasteland [2]. The classic death spiral of an internet forum.
HN started [3] with the idea of trying to avoid that outcome [4], or at least stave it off [5]. Think of it like a complex but fragile ecosystem that needs protecting. Since we all benefit from the ecosystem, we're all responsible for protecting it, much as you wouldn't leave a campfire burning in a dry forest, drive a 4x4 across a mountain meadow, litter in a city park, and so on.
The bonds that hold HN together are weak, because we only have access to tiny blobs of text that are open to misunderstanding. Users don't have relationships that can sustain disruption and still be repaired; the group is too large. Since the organism can't easily repair itself, it needs not to take too many hits in the first place.
Most of the damage is thoughtless rather than malicious. The solution is to become more conscious about the goal of the site and how to further it. This isn't really an ethical question. We're not telling people that they should be good (maybe they should, but who is an internet moderator to tell anyone that?) Rather, it's an optimization problem. We're trying to optimize the site for curiosity [6]. That requires overcoming the default tendencies of the internet, and for that we need to sustain a certain culture.
[0] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
[0.5] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
[1] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
[2] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
[3] https://news.ycombinator.com/hackernews.html
[4] https://news.ycombinator.com/newswelcome.html
[5] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
[6] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...