What? No, why do you like this example? There is nothing special about this case that makes his guilt any more "certain" than any other case. Confessions have been proven to be shit, physical evidence is disturbingly often faked, eyewitnesses are among the least reliable sources of information possible, etc.

The American criminal justice system is designed to reach conclusions, and explicitly not designed to be accurate. That is no system upon which a capital punishment should ever be applied.

I would argue the Boston bombing is a great example of why we shouldn't execute people. He is clearly a disturbed person who needs help, not death.