Justice is about balancing the books. The only way to do that in many cases is through the death penalty.
I've seen the results of studies which say each execution prevents 5, 10 or close to 20 murders. I have no idea how they come up with those numbers, but we do know that when the death penalty was put on hold in the US the murder rate sky rocketed, and when it was brought back into effect the murder rate dropped rapidly.