One of the most common verses cited in support of capital punishment is Genesis 9:6. The warning is more clear in Leviticus 20:4-5. Look at how God warns His people not to neglect His commend to execute those who burn their children to death in pagan rituals:
And if the people of the land do any ways hide their eyes from the man, when he giveth of his seed unto Molech, and kill him not: Then I will set my face against that man, and against his family, and will cut him off, and all that go a whoring after him, to commit whoredom with Molech, from among their people.
There are examples all through Scripture of God punishing the wicked with death. This does not only happen in the Old Testament, because God struck down Ananias and Sapphira for lying about the cost of their land and struck down Herod for allowing himself to be described as a god. Pretending that “thou shalt not kill” prohibits state-sanctioned executions is either a misunderstanding or a misrepresentation of Biblical doctrine.
This is why abortion opponents frame our desire as protecting innocent life. There is no hypocrisy in protecting innocent life while not protecting the guilty from punishment. Even those who oppose the death penalty must recognize that the termination of an innocent life is fundamentally different from the execution of a murderer. (There are abortion opponents who oppose the death penalty, of course.)
Now, are there problems with capital punishment? Yes. Forensic science is too often flawed, and prosecutors too often oppose allowing new evidence to re-open a case after a guilty verdict as been found. We need to be fastidious about making sure that people convicted are guilty, and even more so in death penalty cases. But I do not accept the argument that the death penalty itself is unjust. We are not more holy than God.