You can't report to the moderators, they're just anonymous users that for some reason wants to work for free for Reddit. Often times they have their own agendas, I've used third party sites to show deleted comments that makes it clear some mods support calls for violence against certain groups, depending on which subreddit it is.

I've reported threats of violence similar to what you describes over at https://www.reddit.com/report and they removed it after a day or two, even comments that were highly upvoted.