Are you trying to say that if the death penality would have been an option, then these crimes would not have happened?
Because that would be the only argument of any importance given the whole reason we have a punishing justice system at all is to prevent crime. Not to exact revenge.
I really doubt these people willingly accepted 20+ years in prison, but would have reconsidered in fear of a death penality.
The punishment which acts as the best deterrant while still being reasonable is the most appropriate one. Not the one that 'feels' most appropriate.
As there is apparently (by looking at past data in countries switching from one form to another) little difference in the deterrant effect between long jail time and death penality on hard crime, it makes no sense to apply the latter one, which is riddled with all kinds of problems for society, both economic and ethical.
I agree that it may make sense to make the jail time itself more of a deterrant, but it is hard to get this factor to influence someone BEFORE they end up in jail, ie before they do the crime, which is what we really want.