Great, so who in the legal process do you put to death when an innocent person is wrongly given the death penalty? Do you kill the driver of a car if they have a seizure and hit someone?
Can you quantify the amount of, say, drugs you'd need to peddle to determine it to be life-ruining, knowing fully well that the legal system disproportionately targets minorities or the disadvantaged?
Is sodomy life-ruining, among consenting adults? You could still be prosecuted for it in America as late as 2003. With the protections for abortion rights being in jeopardy in the courts right now, would you have mothers put to death in the wake of a 'life-ruining' miscarriage?
You can't un-death someone if you've got the wrong guy, change your mind about what the appropriate punishment is, laws change, or death is not the desired outcome from the defendant. There will be no plea deals or speedy trials if the death penalty is on the line, straining a legal system that already frequently takes months.
It's barbaric. It offers no opportunities for repentance or redemption. This is not justice and justice does not decide who 'deserves' to die. Life is not so simple, so binary, that 'death' should be an answer to it, and if you find yourself wishing death on people, you need to reflect on your valuation of life.