I think it's important that we don't let what Greed had to say pass unchallenged. It's unthinking, uninformed, badly spelt attitudes like that which lead to discrimination against the most vulnerable and hinder the progress of society. To claim "its the truth" might shore up Greed's own fragile grasp on his self-confidence but it doesn't make what he said any less ignorant.
First of all, disability takes many varied forms. Greed's arbitrary division between "light aspergers" and "property-of-the-state retarded" suggests that mental disability is on some kind of continuum with autism at one end and severe developmental disablity at the other. That's clearly not true. There are cognitive and behavioural disorders, disabilities resulting from physical injury, those caused by traumatic experiences, congenital disabilities, those which require long-term care and those which you probably wouldn't even notice. Just because someone falls into the wide-ranging category that is denoted as "mentally disabled", it doesn't mean they conform to any particular stereotype.
Let's address Greed's specific examples.
He claims, "if you just wave something shiny in a retard's feild of vision you'll blow their minds". When he says "retard", let's be generous to him and assume he's got some sense of what the word actually means, and take it that he's talking about those who are severely developmentally disabled. If someone is disabled in this way, then all it means is that, in one way or another, they will tend not to develop to adulthood at the usual rate. So, they might continue to communicate and understand the world in the way a child would, even after reaching physical maturity. Therefore, to claim that, across the board, developmentally disabled people won't be able to understand magic is the same as saying that a child won't be able to understand magic.
Greed further claims that "an autistic probably won't understand the nature of a magic trick". "Autism" is more properly called "autism spectrum disorder". The reason for this is because it's, unsurprisingly, a spectrum. In the US, it's estimated that 1 in 68 children are autistic. Most of those will be able to be teachers or authors or any other job you can think of. They will certainly be able to "understand the nature of a magic trick". A more interesting question is about whether autistic people are less likely to be fooled by misdirection which relies on social cues. And that question's been answered by research which shows that such misdirection can even be MORE effective for those on the spectrum.
So, it's clear that the kind of ideas that Greed propounds are not only "mean" but just plain old incorrect. Given that belief in those type of ideas holds back, or "retards", development towards the ideals of inclusion and kindness, it's clear that those who hold them should be treated not with anger but with compassion for their problems. It's them who are, literally, retarded.