The dead-ban system makes the life of the moderators much easier by starving trolls who would otherwise create a new account as soon as they got banned. At HN's scale, it is necessary.

This case is unfortunate, though, but I guess that's why the showdead option exists.

I don't know if a negative karma turns on an irreversible ban flag, or if going back to positive will alleviate the ban. If not, I'll write a mail to pg.