Sincere repentance can help to undercut that righteous fever that so spontaneously grips us, making us long for a morally superior violence to come and eradicate the bad. It has, it would seem, ever been thus. This execution is wrong because all killing is wrong. Killing begets more killing and violence begets more violence, always. There's a circle of death, just as there's a circle of life.
Capital punishment is part of the circle of death.
There is no morally superior violence. Capital punishment abdicates its moral ground precisely because it mimics, however unconsciously, the very violence it's sincerely trying to subdue. Jesus wouldn't pull the switch on a death-machine and we shouldn't pull it in his name. We might pray - for Timothy McVeigh and for his victims and their families. He tried to use a violence he understood as being morally superior to make a statement.