So how do I think Evangelium Vitae, which makes limiting the use of the death penalty to cases "when it would not be possible otherwise to defend society," and the Catechism, which uses the language of self-defense (e.g., "defending human lives against the unjust aggressor"), are to be understood as developing the doctrine of St. Thomas?
First, what do I know?
Second, I'm not convinced it's a development that goes beyond what the blessed John Paul II wrote in his encyclical (quoting, as it happens, a paragraph in the Catechism that was rewritten to accommodate what he wrote in EV); namely, that "bloodless means... to defend human lives against an aggressor and to protect public order and the safety of persons... better correspond to the concrete conditions of the common good and are more in conformity to the dignity of the human person."
Moreover, in the same paragraph of EV (n. 56), the Pope does use the term "punishment" in reference to the death penalty three times. So it can hardly be claimed that the fact the Catechism doesn't call the death penalty a punishment means Church teaching has developed to the point that it is not regarded as a punishment.
Third, if you do want to think of the death penalty in terms of self-defense, it's a trivial matter:
Figure 4. The Death Penalty Considered as a Matter of Self-Defense.
See? There's no need to put it under the double effect rubric; St. Thomas long ago provided for the "referred to the common good" justification for killing done in the name of the one responsible for the common good. It's all there.
Granted, CCC 2263 might seem to argue that yes, warfare and execution are both double-effect cases:
The legitimate defense of persons and societies is not an exception to the prohibition against the murder of the innocent that constitutes intentional killing. "The act of self-defense can have a double effect: the preservation of one's own life; and the killing of the aggressor.... The one is intended, the other is not."65
But the internal quotation is from ST II-II, 64, 7, and refers explicitly and only to an individual acting on his own behalf.
That the Catechism should not be read as teaching that the legitimate defense of societies is always a matter of double effect is evident by the fact that doing so is absurd.