The death penalty isn't just about revenge. It's part revenge (and why shouldn't victims get that anyway - it's a long established part of the human psyche and to deny it could be considered unhealthy), part deterrence (maybe not so much in this case, but what about a gang-related murder?), but mostly it's about removing these people humanely from society for the protection of those who do not indulge in the sickest, cruellest acts and do not want to become future victims.
If 'life' meant LIFE, then that would mean an existence without hope of sex, relationships, reproduction, parenting those offspring one may already have produced, boozy nights out/drugs binges, mtbing or most of the other things that give a point to the human experience.
In my honest opinion, that is tantamount to the cruellest torture and obviates any argument anyone could make about the death penalty being 'barbaric' or 'uncivilised' or 'inhumane'.
How far does it differ from the actions of Josef Fritzl? Acts which are quite rightly condemned as all those things.