The doctrine of double effect (DDE) can have two different functions, permissive and restrictive. According to the first function, agents are exculpated from the negative consequences of their actions, consequences that would be deemed illicit were they intentionally chosen. According to the second, agents are reminded that they are responsible, albeit in a distinctive manner, for the foreseeable damages that flow from their chosen actions. Aquinas has standardly been credited with a permissive version of DDE. I argue by contrast (drawing on the treatment of this issue in my Thomas Aquinas on War and Peace, Cambridge University Press, 2017) that the permissive version results from a misreading of Sum. theol. II-II, q. 64, a. 7. Other texts in the same work indicate that he embraced a restrictive version of DDE.
Reichberg, Gregory M. (2019) Restrictive versus Permissive Double Effect: Interpreting Aquinas, Proceedings of the American Catholic Philosophical Association 91 (1): 211–223.