I know what you mean. The Baby P horror came out when DS was a week or two old, and a mother stabbed her baby to death a week later. I was shocked at how personally distressed I was - before, I would have thought it sad, but not felt much more, but once I was a mother of a tiny baby it was horrendous to know other children suffered like that. The nursery case last month actually made me come close to throwing up. So yes, I understand. If anyone did anything that appalling to my son I would want to make them suffer more than they imagined possible, before killing them extremely slowly. But that's why the state is in charge of punishment and not the victim, or their family. Killing is evil, full stop, and an eye for an eye is no basis for a civilised society. Nor is vengeance actually - it serves no practical purpose, and doesn't really help anyone. (It doesn't assuage the pain of the victim. I don't think anything can.) And that's before getting into how often mistakes are made in the criminal justice system - you can't free and compensate a corpse.
Having said that, I do think life should mean life in a larger proportion of cases. Some people are simply not worth the risk of freeing. And personally, I'd far rather be executed than live out my days in a prison, so perhaps it's not the most humane option. I don't really care, tbh, because I think capital punishment is wrong because it makes murderers of us all, and because mistakes are made, not because I care about the murderer per se. I'd rather they rot quietly, in a place they can't hurt anyone élse, without us all being complicit in their murder.