I think there is a lot to be said for Actual Life in prison, rather than 15 years or 25 years. That being said the death penalty is awkward.

Some of the most famous murder cases are famous precisely because of the press. That is to say the press plays up the nature of the crimes and makes the person so reviled that the prosecutors feel obliged to press for the maximum possible sentence. It's not often the merits of the case that decide these things, but rather the budgetary concerns, the public reaction and the effects on elected officials. One of Canada's most famous 'killers' spent 25 years in jail before being found innocent through new evidence (DNA). There have been quite a few such cases with the discovery of DNA evidence, and its hard to believe that the next level of evidence will show the same thing.

People who are alive have advocates to call for such testing. I wouldn't be surprised if DNA evidence would show that a small number of capital cases in the 1970's and 1980's actually involved innocent people. Of course such testing will never get done because no one has their freedom at stake, and the state would be liable for erroneously putting someone to death if they found that they did such. If 25 years in prison erroneously costs between $1-$10 million in damages, I can't imagine what the jury would reward for erroneous executions.

The thing about an advocacy system is that people are routinely "negligent" in the eyes of civil law. When your career is based upon providing evidence for successful convictions, you often don't pursue paths that a reasonable person in the eyes of the law ought to pursue that would eliminate a suspect. The police system is in parts political and like all things political suffers from corruption.