Of people who murdered, were sent to prison, and then murdered again.
Good ol’ Willie Horton was in prison for life with no parole. They just didn’t call it parole when he escaped, broke into a couple’s house, and repeatedly raped the woman and tortured the man (a stranger) with a butcher knife. They both lived, so it doesn’t count. I am sure his two victims are deeply comforted by the knowledge that he is back in the same prison system that let get out in the first place, and can’t bring itself to make sure Willie is no longer dangerous.
Charles Kemp was in prison for murdering his grandparents when he was released. He then murdered nine more women, followed by his mother (who it sounds like was who he was peeved at all along).
Jack Abbott killed a guy, went to prison, got out, killed another guy, went back to prison, and while inside wrote a book (“In the Belly of the Beast”). Norman Mailer read the book, got on his side, and got him sprung. Then Mr. Abbott killed a third time, and is back inside. Maybe if he had been condemned the first time he would be dead by now - and two lives would have been saved.
Henry Lee Lucas went to prison in 1971 for first murdering, then raping the dead corpse of his own mother. He pleaded not to be released, saying he was sure he would kill again. He was right - at least nine times over.
Edward Wien was awaiting the death sentence when Pat Brown, governor of California before Ron Reagan, commuted his sentence to life in prison with no parole. What can be changed once can be changed again, and was, when his sentence was changed from life without parole to life. He was paroled, got a job in an electronics firm, and Pat Brown was wont to point to him as a good example of how wasteful the death penalty was, since Wien was now paying taxes. Pat Brown stopped saying that after Wien was convicted of kidnapping, raping, and murdering a woman he saw waiting for the bus.
A study in Columbia Law Review examined murderers from 1990 - 1994. They found 810 people previously convicted of murder who went on to commit 824 murders (some killed more than once). So in order to argue that the death penalty does NOT save lives, you would have to document at least 825 wrongful executions of innocent people. You can’t; the US has not executed that many people since the death penalty was reinstated.
The best system is that under which the fewest number of innocent lives are lost. Increasing the use of the death penalty would save lives, much more than offsetting the risk of executing the innocent. We still have appeals and judicial review, for heaven’s sake.