The Pope is christian and yet he opposes the death penalty. Oliver North is a christian, and he too opposes it.
The Pope IS the Pope. But he holds a LOT of views that are not Catholic. The just application of capital punishment has been a consistent Catholic teaching, if not itself any dogma. It's based on the very notion of Justice. Remember - this is the 'half-moon' Pope of Malachy's prophecy, friend to Islam and the UN. He's in no position to act as moral authority on anything, frankly. Not this Pope. Hopefully the next.