I think it's a combination of things:

• His videos (as far as I can remember, correct me if i'm wrong) are basically poking fun at other people on the Internet for entertainment purposes, often with an aggressive tone and word choice. Some view it as bullying, some don't like how he's doing it at the expense of others, some just don't think it's funny.
• The subsection of people that watch his videos/know him is generally quite young, and I think are more prone to make dramatic statements (e.g. that crucify line), because Leafy himself is like that in his videos.
• He's quite popular, and with popularity comes haters. That's just YouTube and the Internet.
• He's gotten into some "drama", with his rant videos on Onision, Fousey, Keemstar, etc. So people that support those people may hate on Leafy just because of that.

To be honest, I'm not really a fan of Leafy, but I don't like it when people escalate that into "he should rot and die". But there is a difference between that and "I think Leafy should watch what he says, he might be crossing the line". I think that's valid, but people often escalate their tone and attitude so that becomes "he should rot and die".