To gain understanding of your perspective, does any capital murder reach the level of earning capital punishment? Are there extremes cases, such as insanely cruel torture and death of multiple children "earn" capital punishment?
Of course our moral conscience is outraged by such heinousness acts and we naturally want "justice" by which we usually mean vengeance of some kind of nasty punishment. I'm the same way.
But at least on a moral level, I have to stop and remember that every single heinous thing that was ever and will ever be done was completely condemned, judged and paid for on the cross 2000 years ago. All moral issues were resolved there.
What's left is the civil law and IMO, that should go more to protection than punishment.