Catholic teaching says that the situations in which the death penalty can be used are “rare, if not practically non-existent.” Wouldn’t cases of heinous crimes, such as 9/11, be examples of the “rare” cases?

The test of whether the death penalty can be used is whether society has alternative ways to protect itself, not how terrible the crime was. Life in prison without parole provides a non-lethal alternative to the death penalty. We can’t know whether God has a purpose for a person’s life, even one who has committed a terrible crime and must spend his or her life behind bars.

Does life in prison without parole really work or are those convicted sometimes released?

Life in prison without parole means that the convicted person is not eligible for parole and cannot be released.

I understand that in the past innocent people were sentenced to death, but now that DNA is available, isn’t this avoidable?

DNA evidence only exists in about 5-10% of criminal cases (10-15% of death penalty cases). Where it is available, it is still subject to contamination and human error. The risk of an erroneous conviction is still significant.”

-The above section is from  “Questions and Answers” on the USCCB website about the Death Penalty/Capital Punishment.