So, is a dog human or are humans dogs? When I say 'this is nothing more than putting down a rabid dog" I dehumanize him. Or is he a rat? Or an ape? Or a skunk? A monster?
Dylan Roof is a human being who grew to believe his skin color makes him superior to black people. His mind is poor enough to not be able to see how untrue that it. His life became enough of a failure and frustration that he wanted or needed to kill some inferior beings to satisfy whatever it is in him that murdering innocent lesser beings fed.
He's a ####ed up human being but a human being none the less. If we see him as some animal, then we're just debating what qualifies anyone else as lesser and validating his basic premise.
We're better than that as a society. Or ought to be aspiring to be so. We kill him, out down a rabid dog, we absolve ourselves, one and all, for his actions. Other white supremacists aren't forced to deal with it. People who ignored warning signs get a free pass as he'll never be able to say "Yeah, so and so influenced me, I told this person or that". It's a messy thing. We should not get off so light nor should we want to.