Pope Leo is pushing to end the death penalty worldwide, a move that puts him at odds with American conservatives who want capital punishment kept for the worst crimes.

• The Pope's position is that life in prison is punishment enough, even for mass killers.
• Critics on the right say mercy and justice are not the same thing, and that some crimes earn the ultimate penalty.
• The flashpoint is a case involving a psychiatrist accused of deliberately targeting Americans who trusted him.
• The religious argument is being fought over too: "thou shalt not kill" is read as a ban on murder, not on executing the guilty.

Outlook: Expect a loud fight between the Vatican and American Catholic conservatives, with little chance the Pope changes US death penalty law.