I think that it is more difficult to deal with the long-standing support and Church Father endorsement of the death penalty than God’s direct actions in the OT. Certainly God can do what he wills, and we know that he punishes sin with death. The question is whether or not he permits the state also to exercise the same authority for such offenses that threaten the common good.
On that end I don’t think the OT law is very relevant, since the Mosaic law was an imperfect expression or the moral law, and did not have the respect for the dignity of life that Christ would later display. It also permitted divorce, for example. It is very easy to reconcile OT law condoning slavery and divorce with the Christian teachings, because in Christ we have the perfect expression of the imperfect.
As for the church’s tradition, I think it is fair to say that Pope Francis at least considers that the magisterium did not make any irreformable definition of the death penalty. The fact that he also refrained from providing such a definition however implies to me that he prefers to allow the sensus fidelium to grow alongside his current teaching. I expect that we will see some clarification on this point eventually. In the mean time it is important to recognize Pope Francis’ valid authority and competence to make the judgement he made, while also recognizing that there is a long-standing tradition in the church that is just as valid and just as competent.
It would be a mistake to assume that popes in the past were limited in their understanding of the Gospel and thus ruled erroneously on the death penalty. It would also be a mistake to assume that Pope Francis was not aware of the situation when he determined what he has determined, and for that reason I think he was careful to only speak of the “new” and the “present” rather than explaining anything about the past.