Negative commandment · Lo Ta’aseh #19 of 613
Not to [try to] save a mesit, but rather to see to it that he is executed
שלא להציל המסית אלא עומד על דמו
The verse · Devarim 13:9
לֹא־תֹאבֶה לוֹ וְלֹא תִשְׁמַע אֵלָיו וְלֹא־תָחוֹס עֵינְךָ עָלָיו וְלֹא־תַחְמֹל וְלֹא־תְכַסֶּה עָלָיו
“thou shalt not consent to him, nor hearken to him; nor shall thy eye pity him, nor shalt thou spare, nor shalt thou conceal him:”
The Rambam’s words
Not to [try to] save a mesit, but rather to see to it that he is executed, as [Deuteronomy 13:9] states: "Do not have mercy upon him."
שלא להציל המסית אלא עומד על דמו, שנאמר "לא תחוס עינך עליו" (דברים יג,ט).
Why · Sefer HaChinuch #459
To not save the seducer
“The root of this commandment is written in the previous [commandments] that come about the seducer. And all of its content is like them.”
Where in the Mishneh Torah
Hilchot Foreign Worship and Customs of the Nations הלכות עבודה זרה וחוקות הגויים