St. Thomas Aquinas speaks to the heart of what it means to be human in our relationship with God when he expounds the way of the moral life in his Summa Theologiae. A classic example of the depth of his understanding is evident in his treatment of acts that knowingly kill. His style of writing is succinct and sometimes his ideas are distributed among several texts, but one can mine the riches of his thought with patient reading and reflection. This Article focuses exclusively on the extreme case where a person is certain to die if nothing is done and the only way to save that person is by one’s act while knowing that it must result in the certain killing of another person. Most scholars using some version of the doctrine of double effect interpret Aquinas to permit such an act when it repulses the attack of an aggressor on someone’s life. This Article rejects this conclusion as well as its justification in the doctrine of double effect and proposes a rule that more accurately reflects the texts of Aquinas as he distinguishes prohibited acts from permitted acts. Specifically, it argues that his rule is that, when a person (whether oneself or another) is certain to die if nothing is done and the only way to save that person is by one’s act (as a private individual and not one acting under public authority) knowing that it must result in the certain killing of some other person (whether or not an aggressor), the act is prohibited unless one retains or removes a vital life support from the person killed that belongs to the person saved (whether oneself or another), or unless one ducks, blocks or redirects a deadly force away from the person saved (whether oneself or another who is under one’s charge). If the act does not fit within one of the two exceptions, then it is an attack on the vital life support of the person killed and is a prohibited killing. The first Part explains how and why Aquinas constructs this prohibitory rule and its exceptions. The second Part adds further clarity by applying the rule to several controversial modern-day cases.