Pope John Paul II explains the death penalty

Blessed Pope John Paul II explained the basis of Catholic Teaching concerning the death penalty: 
[Punishment] ought not go to the extreme of executing the offender except in cases of absolute necessity: in other words, when it would not be possible otherwise to defend society. Today however, as a result of steady improvements in the organization of the penal system, such cases are very rare, if not practically non-existent.

—John Paul II, The Gospel of Life (Evangelium Vitae), 1995