Unjustifiable

You cannot punish a crime with another crime.

The death sentence is a crime.  Although the state who does the sentencing, but the state is not the owner of the life of its' citizens. 

Life is a gift from the moment of conception to the natural end of life. 

The death sentence is not the natural end.  Because John Paul II in Encyclical Evangelium vitae said, today society has all the means to render a criminal innocuous without applying the death sentence.

Cardinal Renato Martino - President, Pontifical Council for Justice and Peace