Thomas has been criticized by Alan Donagan (and others) for his use of the principle of double effect (PDE) in justifying defensive homicide. Donagan claims that Thomas uses the PDE in conjunction with a basic moral principle that prohibits us from harming human life. He sees Thomas as using the PDE to reconcile this principle with the traditional Christian doctrine of justifiable homicide in self-defense. Defenders are prohibited from killing intentionally by the basic principle, but the PDE permits them to kill unintentionally. However, according to Donagan, human life is not, as such, absolutely inviolable. When the inviolability of human life is rejected, the PDE becomes irrelevant to the justification of killing. I claim that Donagan misunderstands Thomas's use of the PDE. I then present what I believe to be a radical reinterpretation of what Thomas is trying to do with the PDE. First, Thomas does not regard human life to be inviolable. Only the common good and each person's moral or spiritual good are inviolable, and Thomas does not think that these goods are necessarily harmed by homicide. Indeed, both the common good and the moral good of the victim can be benefited when the victim is a sinner (e.g., an unjust attacker). So it is not the status of the attacker's life vis-à-vis Thomas's basic principle that prohibits a defender from killing intentionally. Thus, the PDE is not meant to permit indirect harm to goods that, because of their moral value, we may never harm directly. Rather, Thomas uses the PDE because he thinks that acting from a certain attitude is wrong even when the consequences of the action are good for the moral values it affects. Thomas believes that persons lacking authorization to kill for the state are capable of killing with a proper attitude toward their victim only if they kill unintentionally. Only publicly authorized persons are capable of killing intentionally from a proper attitude. This is because they have a duty to enforce the law and are therefore capable of killing from (only) felt obligation. Thus, Thomas permits public officials to kill (some sinners) intentionally, something he could not do if he held life itself to be inviolable. I agree with Donagan that Thomas does misuse the PDE in relation to (most kinds of) personal self-defense. But I argue that this does not entail that the PDE is irrelevant to the justification of other kinds of homicide. As Thomas intends it, the PDE may still be relevant to the justification of some kinds of defensive homicide, and maybe (indirectly) to the justification of killing in war. I argue that its relevance to war is not in justifying military actions that kill innocent noncombatants, but rather in determining who is (and is not) justified in undertaking military actions in the first place by virtue of their capacity to do so with proper intention and motivation.