It seems to me that multi-decade prison terms are at least as severe as death. It doesn't seem terrible as long as prisoners are able to kill themselves. But a life sentence in a prison with effective suicide prevention seems far harsher than a lethal injection next week.
The problem with capital punishment is what do you do when you discover after the fact that the person was innocent?
If the justice system were perfect there could be a case for execution in place of life sentences, but it has been shown time and time again that going free depends more on the quality of your lawyer than the facts of the case. Prosecutors will lie and fabricate evidence because the cops did a terrible job and they figure you must be guilty. Even if it's not of this particular crime you're just the criminal type and they'll be doing society a favor by locking you away.
The system is highly classist and racist. It shouldn't be allowed to make such final decisions like killing someone. This makes it impossible to reverse the injustice in the future.
If human life is so precious that it must not be taken, then it's precious enough that it must not be taken by oneself as well as others. If people are worth the chance at life, then a life in a prison cell is as worth that chance as life in a palace.
