Vengeance. It undeniably infects human affairs. Parents observe it as their toddlers navigate playground dynamics: one child hits another; the other strikes back. Domestic feuds and old grudges can metastasize into jealousy, schadenfreude, and—if unchecked—retaliation. Belligerent drivers yield microaggressions that can morph into road rage. In the extreme, assault and murder seem to beg for [...]