I largely agree with you, but to play devil's advocate: a little hostility helps people to check themselves. It forces you to think before you speak and before you post, which means there's a good chance you check yourself before you submit thing. It ensures a degree of quality control, and that's something that I think Hacker News benefits rather than suffers from.
The first forum I joined was one of people who enjoyed ripping on people who said stupid things and who wasted space. I joined there at a pretty young age: I still credit it for teaching me to think about and revise things that I post online. I still remember it fondly, in part because it was a tight community: it scared most people away, but that was good, because it was a site for a niche audience. (Similar, I'd say, to the niche audience that ought to be in Hacker News.)
I've also worked with forums that aimed towards nice: they tended to have a less fanatic userbase, and the people who remained were of wildly varying quality. Nice in excess leads to a downfall.
(I also haven't noticed much unfair hostility until the last month: the election polarized a lot of people, and brought in a lot of attitudes that seem less concerned with new information and interesting articles. Hopefully it will pass.)