I don’t agree with you that letting a prisoner rot (or starve) in prison is exactly the same as executing him. From a halachic standpoint, that is. “
If he is an absolute chayav misah murderer, then keeping him in prison could actually be arguably WORSE than executing him, because the halacha is that someone upon whom the Sanhedrin has pronounced a death sentence, must be taken out to be executed immediately. The halacha is sensitive to the fact that being on Death Row could be emotionally terrifying to the one who knows he is to be executed, and is anticipating it to happen, but not knowing when. Even the worst murderer is to be shown the type of mercy that precludes causing him unnecessary anguish.