Al, you should listen to your wife more.
One of the fundamental problems with our justice system is that we've adopted the death penalty, but our justice system isn't perfect enough for that -- and it never can be. I don't think one wrongful execution can be justified because we got the other bozillion right -- one is one too many. And, at some fundamental level, we should all know that revenge isn't really any solution -- it just makes us feel good, temporarily, but it doesn't fix a thing.
Who's to say the teenagers who shot up Columbine HS are somehow not quite mature enough to have understood, but the 20-something kid who shot up the theater is? That shows your uncertainty with the process. Even if you go all the way and conclude he was fully aware of his actions, and wanted to kill as many people as he could -- is there any question this guy was simply whacko? Lock him up, figure him out, whatever. But killing him doesn't fix a thing. Surely you're just spouting off in the heat of the moment.
Overcrowded jails, endless appeals, money spent, whatever -- they're the symptoms of a (mostly) fair and civilized system. We're lucky we've got it.