This chapter discusses the fundamental question of the defense industry’s role and legitimacy for societies. It begins with a parable of a psychopath doing something self-serving that has beneficial moral consequences. Analogously, it is argued, the defense industry profiting by selling weapons that can kill people makes it useful in solving moral problems not solvable by people with ordinary moral scruples. Next, the chapter argues that while the defense industry is a business, it is also implicated in the security of its home nations. In addition, its products can be used for good or evil. Thus, it must navigate conflicting values, with the result that the rules often thought to rightly regulate the industry—codes of business ethics, engineering ethics, procurement ethics, plus laws of the land, and dicta of conscience—need supplementing by general moral reflection. This chapter argues for a principle that tells how to decide when to violate the rules, for a theory of rational choice on which choosing by this principle is rational, and for teaching that theory to defense industry employees to ensure their ethical behavior.