Actually, it happens all the time. Short of absolute solitary confinement, it is impossible to prevent one inmate from ordering the killing of another inmate, a guard, or any civilian on the outside.
So you value human life. So do I. Unfortunately, valuing human life does not nearly resolve our dilemma, which, I remind you, is about the existential reality of choice, not death. Like it or not, we are forced to choose between killing the incorrigibly homicidal criminal or risking the possibility that he will continue to harm others, even if only a prison guard or another inmate. Of course, it would be absurd to suggest that the life of this prison guard or inmate is somehow less existentially valuable than that of the incorrigible criminal at hand. Yet that is what you are ultimately suggesting here.
You do realize that death row inmates are held under much tighter security than lifers? There are also comparatively few of them, and for many of them, the DP appears to have some degree of rehabilitative effect now that the existential reality of death is pressing down upon them so heavily. Many appear to have serious concerns about what will become of them in the hereafter.
Thus, you are looking at the wrong "continuum of possibilities." The problem does not lie among death row inmates. The problem lies among inmates sentenced to LWOP. Lifers are notorious for committing all sorts of atrocities while serving out the length of their term.