The problem with this is that the laws are interpreted to afford convicted criminals a lot of leeway. I recently watched a video where a man was sentenced to death for raping and killing a child, and he looked pretty calm, because he knew he had infinite appeals and, even if none of them managed to overturn his sentence, he could plausibly drag his case out until the end of his natural lifespan. And sure enough, that guy has appealed.
This almost certainly wasn't the original intent of the Framers of the Constitution, seeing how they lived in an age where a man could be sentenced to hang for a robbery and then have the sentence carried out on noon of the following day, and they were apparently fine with this. But the currently in vogue interpretation of the law would be very hard to replace.
A potentially easier alternative would be what I call "Engagement Doctrine". Basically, there is a window of time between when a person begins to act in the commission of a violent crime, and when they've clearly acted to disengage themselves from said activity in a way that a reasonable person could discern, during which their right not to be killed by anyone is forfeit. If this doctrine was recognized by the courts, and if the police were to become highly efficient at finding and killing criminals during said window without a large number of miscalculations, it could do far more than our nerfed death penalty to keep our streets and communities safe.
