There was a time when executions (whether by hanging, guillotine, or whatever method) were highly public events, often held in the "town square" at a time when most people could attend.
Parents often made it a point to bring their children to witness such events. Sure, it was an extremely unpleasant sight for children, and one they would probably never forget.
That was the whole point. See it. Don't forget it. This is what will happen to you if you grow up to commit heinous crimes.
These days, nobody can be allowed to "see" an execution. And some brutal child-raping and murdering psychopath cannot be allowed to endure the slightest amount of discomfort. He must be painlessly "put to sleep."
It's possible that if there were more stark, more public ways of dealing with these horrendous monsters, fewer children would grow up thinking it was acceptable to be just like them.
As George noted, this Roof perv was not the least bit remorseful. He probably is, in his mind, a noteworthy person who is "famous" and who has probably gained heroic status with the KKK-type crowds. He probably never expected to be executed, but instead living a long, secure life in prison where he could enjoy his "celebrity" status.