I am a firm believer in St. Augustine of Hippo's concept of a "just war" (and that includes
killing) and the four conditions that must be met:
1. the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;
2. all other means of putting an end to it must have been shown to be impractical or ineffective;
3. there must be serious prospects of success;
4. the use of arms must not produce evils and disorders graver than the evil to be eliminated.
IMO, no war in which the US participated or initiated since WW II have met these conditions.