So... forgive me for dropping this on this thread like three years later, but one of the dominant narratives is a lil off, and I feel the need to Correct The Record
™ because the true story is even better.
So, yes, MM doxed some random kid for a rape threat. But the kid in question didn't issue said threat. Basically, channers were fucking around and having a laugh over how hyper-sensitive Tumblr could be and how quick to life-ruination they were, and decided to play games.
So some channer took some random kid's face (could have been one of his enemies, who knows) and used it to send a rape threat to MM. MM (presumably a grown adult who should have taken five seconds to think the whole thing through) immediately sends her Tumblr mob to ruin the (again, uninvolved in these shenanigans) kid's life. When MM realized how badly she'd fucked up, instead of taking pause to reconsider whether her own actions were appropriate, she blamed 4chan for the shit the kid received (that MM had called for him to receive).
Unfortunately the best documentation I could find on the quick for all of this is this Imgur/Reddit image macro, but it does an almost passable job: