It's a fact. Under the anonymity of the Internet people grow balls. Some people, cowardly otherwise, use this 'strength' to spread the misery around. Unfortunately, with the way YouTube has their comments set up they're making hate easier than ever. The most responded to comments get shoved to the top of the video page. Those comments tend to be the most demeaning and YouTube sustains the reputation of hosting some of the most hateful comments on the web.
You're probably thinking what I usually do. Don't be on YouTube if you can't laugh at it. We can laugh at the offender's misspellings and how stupid and ridiculous their hysteria is but now we can't fire back. The more responses the person gets the higher their comment gets bumped up to the top of the list.
I think this creates a mob mentality of insulting the video host. I can ignore it to a point. If you can't take ridicule and develop thicker skin don't be in the public eye. Still, it's one thing to make fun of someone's appearance or how they present the topic but a group of people telling someone to go 'f*cking kill themselves'! Especially someone who clearly might be depressed. I find the growing numbers astounding. The one thing I can think of doing is ignore the trolls and post something nice to the host before leaving their video page.