I think that what Greenman reacts to is is a moral belief that hatred and revenge for evil doers does nothing to stem the actual evil that exists in the world. He used the assumption of a low IQ to make that point. How do you cure the stupidity of low IQ people for whom forethought about the consequences of evil acts won't happen in their thought process.
"So lets just all not wring our hands is hopeless and painful frustration, Let's act out a desire for retribution and let them rot in prison."
That doesn't agree with Greenman's moral character in my opinion. I base my conclusion of the fact that he suggested there should be some sort of attempt in our system of justice that might consider helping people who never grew up with an ability to not act in ways that are destructive and socially impermissible.
There may be better and worse ways to make such an argument, like not assuming someone has a low IQ, but it is, in my opinion, an assumption that if true makes a mockery of the justice system so many vengeful people who desire who are unable to conceptualize his moral concerns.
I don't see then, frankly, an apology for the evil of others in what Greenman has said, but an appeal for something better than vengeance for criminal acts. To my way of thinking that is a profoundly liberal ideal.