If I insult another user on Hackernews, how is Ycombinator resposinble for that?Your example is a difficult one because only the person who the derogatory comment is aimed at can decide whether or not they're insulted. Whether or not something is insulting is up to the person it's aimed at. The same goes for things like negative comments, stupid comments, copyright on a derivate work, etc. Whether those things are actually bad is a matter of opinion, and each party probably takes a different position. Consequently it's different situation, and not really relevant here.
A better analogy would be if I were to invent a piece of plain text malware and posted it in a Hackernews comment. Would YCombinator or HN have any responsibility to remove it, or should they just let it sit there? I contend that when something is actively harmful the publisher has a duty to protect visitors by removing the content or limiting it's impact. (And HN has some awesome moderators who do exactly that in very extreme cases, plus users here can flag things to hide them when there's a consensus, so it's not really like HN is completely free of 'censorship')
Plenty of people take the opposing view that platforms shouldn't get involved. There are two sides to most arguments. I'm slightly on the other side to your position.