Is this what is meant by "justifiable homicide"?

Glenn Hunter, who seems to be a death penalty advocate, wrote yesterday on the Frontburner blog (I've been busy so I just now caught it):

"The penalty’s underlying principle is simple: Human life is precious–so precious, in fact, that if you take someone else’s, you forfeit your right to yours. In other words, you pay the ultimate price–just as you made your victim pay."

OK, Glenn, so answer this: The executioner has just taken someone's precious life, so, using your argument, why shouldn't he now forfeit his right to his?