I think you're largely right, but IMO the difference between what happens online and what happens offline is that you have to look people in the eye offline and you may encounter them randomly in person. There's a moderating function that generally (not always) keeps the heat down.

Same for pile-ons. I'm not so naive to think that they never happen offline, but there's a higher cost to doing them than there is when all you have to do is write 140 characters to dunk on someone you won't even remember in two weeks.

People that write on the internet are probably just on the average less sane. There are prevalent types on the interwebs that I just never meet outside.

Are they a different set of people online and off, or a just a different set of behaviors for the same people?

Reddit is far worse than human nature, I believe. The upvote and downvote system favours people that are authoritarian (as a personality trait). There is this sect like wibe nowadays on Reddit.

I don't think voting is related. There are some subreddits that do have quality content, but this requires a substantial moderation effort. The *chan sites are even worse than Reddit, and there's no user voting AFAIK. Slashdot had a voting system before Reddit, and the discussion there is mostly civilized. Usenet didn't have a voting system, and the discussions were civilized before Eternal September, when users generally followed a netiquette. HN itself has limited voting and civilized discussion, but the site could get overrun at any point if the userbase grows rapidly, and new users decide not to follow the rules. There have been Reddit-type comments here for a few years now, and it seems to be increasing.

So it's a matter of community morals and values, and laborious moderation efforts to sustain it. Once the community grows large enough, moderating content becomes unmanageable. All large social media sites struggle with this, as they literally can't keep up with the amount of content. This is a scale problem that is unique to the internet.

Reddit is arguably worse than 4chan on this front. I don't know about altchans.

Imageboards push actually contrarianism to some degree as it gets more replies (the only metric equivalent to a voting system)

No I think the upvote and downvote system gives psychotic vibes. The normal disagreement does not exist (outside some niche subs maybe).

I dunno but Slashdot but you can have a voting system if users are not using it to silence those they disagree with. Not showing the score goes a long way of keeping it sane, like HN.