本文以乾隆三十七年（1772）臺灣的一樁兄弟相殺案為焦點，考察十八世紀中葉以降清律殺姦「勿論」的適用範疇、成立之要件，及其實踐上的種種細節。清代州縣官在實務審判上，呼應「殺死姦夫」律一貫所重的「姦所」、「登時」、「姦通」三者；試圖從空間、時間、及屍身勘驗的辯證關係中，獲取「勿論」的判定跡證。矛盾的是，仵作檢屍往往直以毆殺傷處為相驗標的，邏輯上不免流於證殺而不證姦之失。一樁包裹著尊卑相犯、手足相戮的殺姦情事，服制關係多少構成裁判上的挑戰。審判案卷在紀錄書寫的字裡行間，透露了律例體系與裁判官員試圖在遏止姦情、維持人倫秩序及對兇嫌（本夫）的同情理解之間，取得平衡並作出適切的處置。

This paper commences with an examination of a case of fratricide committed in Taiwan in 1772 and goes on to investigate the occasions, terms, and details of when jurists adjudicated according to the statute regarding “killing adulterers with impunity” 殺姦勿論from the middle of the eighteenth century onwards. In practice, local magistrates aimed above all to render their judgments in accordance with the law regarding “killing an adulterer.” Accordingly, they inquired whether or not the husband had caught his wife (and her partner) in flagrante delicto, and whether the husband had killed the adulterers “on the spot” and “at once.” Their deliberations took into account the timeframe, locale, and post-mortem examination in deciding whether or not to declare a husband innocent. The wounds and injuries on the corpses were usually examined by coroners; however, they rarely mentioned potential clues of adultery. As a result, the autopsy report always revealed the fact that the husband had killed someone (and his own wife), but failed to present evidence of illicit sex. Given that this case of killing an adulterer involved hierarchical fraternal relations, the mourning system more or less challenged the husband’s right to kill, presenting another adjudicative dilemma. Reading between the lines, Qing judicial archives reveal that statutes and jurists tried to deal properly with complicated situations, and achieved an equilibrium between the prohibition of adultery, principles of human relations, and sympathy for the killers (husbands).