Deleting dissenting comments seems to in general be a good signpost for when someone has started behaving badly, in that it seems to happen eventually in almost every bitter online debate, regardless of topic (i.e. from discussions of things with serious real word implications, like this one, to radical feminist criticism/wank over Joss Whedon, to ship wars), often from someone who's losing the argument.

If I ever host a bitter comics-canon fight on my lj again (or any other very contentious discussion), and for some reason decide it would be a good idea to delete comments that disagree with me/my friends/whatever (outside of someone asking me to remove x comment containing y piece of personal info or whatever), I hope someone calls me on it.