A lot of them are just trolls/spammers , however there are a number that I feel sorry for because they post relevant and sometimes long comments on threads and I imagine that they wonder why they are not getting any votes or replies.
I think it's easy to get off to a bad start on an online community simply because you don't understand it's ethos rather than outright malice.
The rationale is that trolls and spammers will usually register new accounts upon being notified of a ban, but a hellbanned troll/spammer might continue to use the hellbanned account without realizing that their posts are ignored.
* The test for a comment being auto-killed (marked [dead]) is "(or (ignored user) (< (karma user) comment-threshold*))"
* comment-threshold* = -20
* Karma is still updated on item authors if their post is dead - as long as the tests on the person doing the voting pass.
* Once you have the ignore flag set, only administrators can remove it - it is independent of karma.
* You get the ignore flag set for posting a link to a banned site, posting a comment containing banned text, or if one of your posts is blasted or nuked by an administrator.