Ugh, this is sad indeed. I'm not a big watcher of youtube reviews, I prefer to read instead, as it allows me to insert my own voice, tempo and inflection. However, I have watched a few youtube vids, and Marc's were by far the most informative, especially with regards to niche reviews.
I could see violence coming into play with respect to people overdoing application, repeatedly, to piss somebody off, but on a more intimate level. I can't imagine anyone threatening anybody over fragrance reviews, but the world is a huge bag of mixed nuts, and I guess there's all kinds out there!
Best of luck to Marc, insanity is not a pleasant thing to deal with...