And what exactly is killing him going to teach anyone? Nothing. Out of sight, out of mind, right?
The notion that we can justify the state killing anybody under any circumstances is morally repugnant.
Imprisoning him for life won't teach racists and their sympathizers/those who've drunk the Kool-Aid anything either, at least to any discernible effect. Note, I don't really care what happens to Roof so long as he never comes out of prison ever again. If his death penalty were appealed, I'd probably roll my eyes but I wouldn't be angry or anything.
But I am tired of people trying to wrangle secret wisdom and 'debates' and 'conversations' from white perpetrators of hate crimes while ignoring the wisdom from the actual victims of these crimes. I'm tired of black abuse being relegated to a fucking curiosity, an academic exercise, a chance for people to grandstand and concern troll about mental illness, when they don't extend this same kind of novelty towards other people who commit heinous acts. Maybe if we collectively had this kind of concern for out and out racism and white supremacy, we'd actually be farther along than we are right now.