Yesterday, two people were executed. One was probably guilty. One was maybe not. But even if they were guilty, is justice served by execution?
I say no. Killing is a bad thing. Regardless of who dies, death is bad. Killing may sometimes be the lesser of two evils (for example, self defense), but that doesn't make it good. Another solution that didn't involve death would be better.
Moreover, killing accomplishes very little. The victims of a murderer aren't brought back to life by his execution. It doesn't undo the damage that's been wrong. The only positive things I can think of are that it prevents the criminal from committing future crimes, which can be done through other means, and that it gives the survivors revenge-inspired happiness, which isn't really something that should be encouraged, in my book.
Two wrongs don't make a right. Justice isn't achieved by hurting someone who hurt you. Justice is achieved by healing hurt and righting past wrongs. Sometimes that's not possible, but that doesn't make vengeance right.