The Church teaches that when truly dangerous criminals cannot be incarcerated, such actions may be moral. It has to be justified. The problem is that the criterea are subjective, so there's really no black and white answer like in abortion.
No, he does not believe it is acceptable. But he can't say "As the Vicar of Christ, I say that capital punishment is always evil." Because that would be an alteration of doctrine, which is impossible in the Catholic Church. He can say that at this time in history, in his opinion, the death penalty is not necessary and should rarely if ever be used. And the Pope does not let governments off the hook either. He advises them of his personal beliefs all the time.