Sanhedrin 57B

נהרג עליו.

he is executed for killing him even though he acted in self-defense, and a descendant of Noah is also killed for this.

אשכח רבי יעקב בר אחא דהוה כתיב בספר אגדתא דבי רב: בן נח נהרג בדיין אחד, ובעד אחד, שלא בהתראה, מפי איש ולא מפי אשה, ואפילו קרוב. משום רבי ישמעאל אמרו: אף על העוברין.

§ R' Ya’akov bar Aḥa found that it was written in a book of Aggadot in the study hall of Rav: Contrary to the halakha with regard to a Jew, a descendant of Noah is executed on the basis of the verdict of even one judge, and by the testimony of even one witness, and without being given forewarning before committing the transgression. He can be judged or testified against only by the mouth of a man and not by the mouth of a woman; but even a relative may judge his case or testify against him. The rabbis said in the name of R' Yishmael that a descendant of Noah is executed even for killing fetuses.

מנהני מילי? אמר רב יהודה: דאמר קרא, ״אך את דמכם לנפשתיכם אדרש״, אפילו בדיין אחד.

The Talmud asks: From where are these matters derived? Rav Yehuda says: They are derived from that which the verse states: “And your blood of your lives I will require; at the hand of every animal I will require it; and at the hand of man, even at the hand of every man’s brother, I will require the life of man” (Genesis 9:5). It is derived from the term “I will require,” which is stated in the singular, that a descendant of Noah is executed on the basis of the verdict of even one judge.

״מיד כל חיה״, אפילו שלא בהתראה. ״אדרשנו ומיד האדם״, אפילו בעד אחד. ״מיד איש״, ולא מיד אשה. ״אחיו״, אפילו קרוב.

It is derived from the phrase “at the hand of every animal” that one is executed even without forewarning, as an animal certainly cannot forewarn someone. It is derived from the phrase “I will require it; and at the hand of man,” with “I” stated in the singular, that the sentence is issued on the basis of the testimony of even one witness. It is derived from the phrase “at the hand of every man,” that the judgment and testimony must be at the hand of a man, but not at the hand of a woman. It is derived from the term “his brother” that the testimony of the witness is accepted even if he is a relative of the defendant.