Basically people are mad at matpat because he made a video on the Editor Wilbur Arg after Wilbur told him not to. Not only that, but a lot of the theories he proposed were kinda ableist. As someone who had a very long hyperfixation on the arg I can tell it wasn't all that well researched either, you should watch Red Herring's video for a better understanding of the arg.
Also most of the theories presented in Matpat's video were debunked by Wilbur himself.
I have watched red herrings video, I'm actually a fan of the Editor Wilbur arg and have watched alot of theories it's just that I was so confused by matpats theory and video I didn't understand why people were mad. I have since been informed by friends of mine and people I follow on Tumblr and in full support of the death penalty against Matpat