What error? The pope is entitled to his personal opinion. He is not proclaiming this as infallible teachinng. He can't really, anyway. The matter of sentencing criminals is a matter of prudential judgement, since so much depends on circumstances. He didn't rule out the death penalty in principle.
Of course, the “Vicar of Christ”, the one in the seat of Peter, the leader of your entire organization, the Substitute for Jesus on Earth, just speaks his opinion...like pope pius IX in the Quanta Cura:
“These false and perverse opinions [of democracy and individual freedom] are so much the more detestable, by as much as they...hinder and banish that salutary influence with the Catholic Church, by the institution and command of Her Divine Author, ought freely to exercise, even to the consummation of the world, not only over individual men, but nations, peoples, and sovereigns.”
You gotta love these guys...