Thall Shalt Not Kill*. Just because capital punishment's use is clearly racist, it's ineffective as a deterrent, and it costs us more than putting someone in prison for life, I see no reason why the state shouldn't practice vengeance.
And why draw the line at 18 years old? As you can tell by watching TV, kids today are getting more and more violent. If killing a few kids makes us feel safer, we should put aside the fifth commandment and flip the switch.
*Government employees exempt in applicable states.