He is EDM's evil stepchild.
Many hail Deadmau5 as "my fave DJ!" Some refer to the King Rat as the "Troll of Trolls." Others whisper "asshole."
For years, Joel Zimmerman, the human who wears that famous rodent helmet for a living, has been cultivating a reputation (mainly via Twitter) as a perpetual, unrelenting jerk. He might bitch about the price of tickets for his own club gigs. And he might slander any and every EDMer for any and every reason. And he will certainly sneer at the suggestion that he should be overly nice to someone just because that guy's undergone gallbladder surgery.
So last night, as the Mau5 took Ultra Music Festival 2014's main stage in place of the ailing Avicii, it was probably inevitable that he'd piss off a lot of people.
See also: Ten Things Deadmau5 Hates Most About EDM