That's the problem with the death penalty: There are vanishingly few situations where we can be 100%, absolutely clear on whether a person did something that warrants permanently removing that person from existence.
We are a nation of laws and we are also a nation of fallible human beings. If we could be completely certain, then perhaps I would be OK with the permanence of death. But we can't, so we shouldn't be killing criminals in the name of the people or the state. Because what happens if we are wrong? What happens if we are wrong 138 times[0]?
Why have we forgotten that "[i]t is better that ten guilty persons escape than that one innocent suffer?"
0 - http://tcadp.org/wp-content/uploads/2011/01/TXDPFactSheet01-...