When I was much younger, I thought the UK should re-introduce capital punishment. But as a deterant, it doesn't work. If it did, then murders would stop overnight in places where the death penalty still exists. As punishment? No; because a punishment has to be a learned thing. A punishment where the criminal is in a state of mind he thought death was an escape would be a punishment that would deter. What that is, is open to debate.
This case with Roof is different in that he's wanted to be executed; so he should be denied it. He's calling the shots. He's wanting the publicity, the narcissist should be put in a cell, on an island, alone.
As a footnote; I watched a documentary about an ex executioner of Georgia. He'd been responsible for dozens of criminals. At the time he really believed the punishment was justified and proper. But now, many years later, he thinks the opposite. He feels it serves no purpose whatever, other than to satisfy the vengeance of the population.
I still react at various cases, especially with kids, that they 'should string the bastard up'. But the red mist goes and I still come back to the punishment should be banishment with no parole for the most terrible crimes.
The Death Penalty works as a deterrent, at least a specific deterrent (the guy put down will never commit another crime.) Giving somebody "life" allows them to often commit more crime, even in prison (rape, murder, attacking guards, fights, drug dealing, etc.) And the costs to house*. As a general deterrent? The death penalty isn't likely a very effective one, at least for murder, (who knows if it was implement for other crimes, maybe it would be a deterrent?) since most murders are done in the heat of passion, impulse when committing another crime, etc. The only problem I have with it is that we don't have perfect information, in many circumstances and, depending on the crime, victim, place, accused there might not be a real fair trial (the whole tax-payer funded State, or Crown, against some poor guy with an appointed public defender just out of law school.) Having said that, if someone witnesses a violent crime or rape at the scene, so there is no question of what is happening and who is the perp? I am not against stringing them up or shooting them right there on the spot and would acquit anybody who did so if I serve on the jury or not indict if sitting on a grand jury.
* I like the old idea of abandoning them on some island. I'll bet sociologists, and the general viewing public, would like to see what happens, how a society would be organized, could they survive, cooperate, etc.