An English translation of the full text of the letter can be found here. The third paragraph could not be a stronger exposition of the obligation imposed by the Council of Trent that Holy Scripture should be understood and expounded in accordance with the Holy Fathers and doctors of the Church.
But the fourth paragraph qualifies this in a quite remarkable way (the part translated below is taken from James Brodrick's two volume 1928 biography of Bellarmine rather than the website linked above):
If there were a real proof that the sun is in the centre of the universe, that the earth is in the third heaven, and that the sun does not go round the earth but the earth round the sun, then we should have to proceed with great circumspection in explaining passages of Scripture which appear to teach the contrary, and rather admit that we did not understand them than declare an opinion to be false which is proved to be true....I do not think any one today would insist on interpreting the passages from Genesis, Psalms, Ecclesiastes and Joshua according to the consensus of the Fathers and doctors as it existed in 1615.
Applying this to Catholic teaching on the death penalty:
1. Though there is a consensus in favour of the (conditional) legitimacy of capital punishment, I am finding it difficult to find a point at which one can clearly say it became defined teaching. That being the case, the freedom then still exists, in the sense in which Bellarmine suggests, that the cited passages of Scripture might be understood differently without it becoming a matter of heresy. (Indeed, I suspect that the passages of Scripture where the Church has defined one particular understanding rather than another are relatively few.)
2. In this case, the use of reason being suggested by Pope Francis lies in the field of the humanities, and, in particular, our understanding of the nature of the dignity of the human person. One can look, as does Pope Francis ("... No man, “not even a murderer, loses his personal dignity...”), to the inalienable nature of the rights of the person (cf the preamble to the UN Universal Declaration of Human Rights). And one can also look, as does Pope Francis again and as is recognised to an extent in the Catechism of the Catholic Church n.2267, to studying the injury to human dignity represented by the death penalty (the writing of Sr Helen Prejean provides an evidence that can be considered in this regard).
3. That a definition against the death penalty could constitute a genuine development of doctrine appears to me, in the light of the considerations above, quite possible, as Pope Francis himself suggests.
4. But I suspect that a final definition one way or the other may not be forthcoming and, rather like the question of conscientious objection and the legitimacy of military service in the teaching of Vatican II (cf Gaudium et Spes n. 79), the two strands of Catholic life with regard to the question will continue to exist side by side.