The Talmud explains that Phineas fulfilled the law כל הבועל ארמית קנאין פוגעין בו. One who engages in sexual immorality with a gentile woman allows a zealous Jew to perform a para-legal action and kill the perpetrator. However the גמרא qualifies this ruling that the decision has to be self-inspired. If however one comes to the Jewish court, בית דין to ask what to do they do not rule that he may kill the perpetrator. The zealot has to catch the perpetrator as he is committi