I don't disagree with your premise, but using a rapist in an argument for the death penalty is fatally flawed in that there is no that I am aware of that allows the death penalty for rape. It is reserved for first degree (premeditated) murder, under the sound moral principle that once someone has made the willful decision to deprive another of life they forgo their right to live. It doesn't apply to unintended or accidental murder (i.e. manslaughter). Applying it in the context of rape (while on its face a compelling contrast in positions), is ultimately self-defeating for this reason.