B'nai Noach are forbidden to murder innocents. They are also required to establish courts.
If the Gentile court convicts the murderer, do they need to execute the murderer, or does performing the penalty apply only to Jews?
בֶּן נֹחַ שֶׁהָרַג נֶפֶשׁ אֲפִלּוּ עֵבָּר בִּמְעֵי אִמּוֹ נֶהֱרָג עָלָיו.
A non-Jew who kills someone, even a fetus in its mother’s womb, is executed.
More generally, ibid. 9:14:
וְכֵיצַד מְצֻוִּין הֵן עַל הַדִּינִין. חַיָּבִין לְהוֹשִׁיב דַּיָּנִין וְשׁוֹפְטִים בְּכָל פֶּלֶךְ וּפֶלֶךְ לָדוּן בְּשֵׁשׁ מִצְוֹת אֵלּוּ. וּלְהַזְהִיר אֶת הָעָם. וּבֶן נֹחַ שֶׁעָבַר עַל אַחַת מִשֶּׁבַע מִצְוֹת אֵלּוּ יֵהָרֵג בְּסַיִף.
What must they [non-Jews] do to fulfill their requirement regarding the Law of Justice? They have to set up magistrates and judges in each district to judge the people with regard to these Six Commandments; and they must issue warnings (about them) to the people. A non-Jew who violates one of the Seven Commandments is executed by means of the sword.