I'd call him a person of interest. I mean, he apparently was the guy who bought the guns, ammo, and body armor whose actions and attitude bothered the gun shop owners sufficiently to where they called the FBI. The FBI did, it appears, have him on their official "Patsy" list and considered goading him into making an attack. There's the father connection, the Wackenhut connection etc. He may just be an official dead patsy. Or maybe he's guilty. But yeah there's no real evidence he was "The Shooter" that WE are privy to at least, and I personally am disturbed at how people are so quick to accept that he was, and to have it cemented into their minds.
I mean, 9/11 hijackers, the stuff left behind in Atta's car, the miraculously appearing (and undamaged) passport, Lee Harvey Oswald apparently having an imposter go to Mexico to implicate his presence there, and yes, the still-mystifying and clearly bogus San Bernadino incident -- where I, like you, believe that poor couple were completely innocent and unwittingly set up and executed without having a clue!
Yeah the burden of proof is on them and I agree the bar should be set high. Why believe anything they tell us? Did anyone even see Mateen's body? The stories of his demise seem as sketchy and vague as the other parts of the story. Where's his body? Buried at sea so as not to upset the Muslim community?

"He who wounds the ecosphere literally wounds God" -- Philip K. Dick