You see them because you've selected "showdead" on your profile page. They are dead because the person exhibited a lack of understanding of the mores of the community, and either posted spam, or egregiously violated common courtesy in comments.

Sometimes, I imagine, users may get auto killed prematurely; if their first submission appears to be obvious spam, for example. But, I wouldn't think that would happen often.

Check out this user's threads page with showdead on: http://news.ycombinator.com/threads?id=wernerv0nbraun -- all of their comments get deleted.

If the site has a honeypot system that shows the autodeleted comments only to the user that made them, that's clever, if a little much.

If it just instantly deletes them, that's lame, it'd be better just to ban him.

If this is being done by a moderator manually, that's fucking awful, and the moderator should be banned.

Notice his comment 51 days ago that led to the autobanning. This wasn't just a random act of vitriol: there was something that led to the ban happening.

PG said before that his solution with fighting trolls and spammers is to make it impossible for them to see that they're being killed. That way, unlike banning, they don't make new accounts.