Turn on showdead to see downvoted comments. Usually they are low effort and low value; you’re not missing much.
I did not know that. It is amazing how perception of HN changes drastically once you turn this flag on. It is not as extreme as "turning into a completely different website" but certainly it becomes less interesting. There is a sizeable amount of toxicity.
Yes. That's one cost of having a large, open, optionally-anonymous internet forum. We can't stop such posts but consigning them to a separate partition seems to work ok.
If you wish to see such stuff, turn on "showdead" in your profile settings.