Lowering yourself to their level only hurts you. Short of a miraculous change of accepting Jesus and repenting, their eternal trajectory is beyond all human understanding of pain. No joy, hope, light; only agony for eternity. It makes no sense, in light of eternity, to physically torture them.
It is impossible for us to revel in any earthly punishment the killers may receive because we believe that the worst of it only pales in comparison to the first nanosecond in Hell. For the unbeliever or uninformed there is no such comprehension so they are resorting to what makes sense to them. We can only try to illuminate the unseen as we have come to understand it, as you have been rightly endeavoring.
Thank you for posting your thoughtful insight. For Christians who understand how deeply Christ loves each of us including the two killers we can only hope that they will repent, for Christ’s sake.