Holocaust denial is both factually incorrect, and offensive. How should it be flagged?
Should it be flagged as "rude and abusive", or should I use "other" and say "lacks citations"?
I'm worried that flagging it as "rude and abusive" is like getting Al Capone for tax evasion - it wouldn't apply if someone, say, posted pseudohistorical content that wasn't offensive.
Why are you worried that it won't apply to non-offensive pseudohistorical answers? Just downvote those. Deletion should be used for offensive or rule-breaking post, not as a tool against incorrectness. Similarly, lack of citations is not a reason for deleting a post.
In this case, if you believe the post is offensive, simply flag it as such (i.e. rude and abusive).
