One of my suggestions to solve the mean-dumb-upvoted comment problem:

if a comment is truly so mean or dumb that the moderators choose to kill it, allow them the option of applying a karma penalty to anyone who upvoted it (which should come with an automated warning message so people know why their karma suddenly dropped). It's, in a sense, a way of voting on votes.

People might still upvote such comments, but at least then they'll know they're taking a risk by violating HN standards.