The weird thing about saying "Do not save those who took another's life" is that you are saying, "The person took a life. That's bad. We should punish them for the bad thing." Then saying, "Let's take their life to punish them." (not saving a life you could being essentially the same as life taking, to me, here).
To me, you are undermining your view that life-taking is bad, by engaging in it yourselves. To me, this is a paradox. You seem to be standing up for your belief in the badness of life-taking, by doing something against those who do it, then you do something which goes against the very thing that you are trying to protect--which, to me, makes it seems like you are merely pretending to protect.
So, to me, their moral claim (?) that they are somehow "better" than this life-taker, and therefore have a right to say/do this life-save withholding, is completely undermined by their action of said withholding (or proposal to do it).
On the other hand, my ape-brain very much endorses the logic of revenge and sees no problem with an eye for a eye. It will teach people a lesson, let them know they can't mess with us, and let them know there are consequences if they try. Hopefully, my ape brain says, that will make us safer and rest easier, in future.
But the logical/rational part of me trying (but maybe failing) to consider this from an ethical or moral point of view, says, "Nah, that doesn't work."