Commentary for Sanhedrin 8:7
וְאֵלּוּ הֵן שֶׁמַּצִּילִין אוֹתָן בְּנַפְשָׁן, הָרוֹדֵף אַחַר חֲבֵרוֹ לְהָרְגוֹ, אַחַר הַזְּכוּר וְאַחַר הַנַּעֲרָה הַמְאֹרָסָה. אֲבָל הָרוֹדֵף אַחַר בְּהֵמָה, וְהַמְחַלֵּל אֶת הַשַּׁבָּת, וְהָעוֹבֵד עֲבוֹדָה זָרָה, אֵין מַצִּילִין אוֹתָן בְּנַפְשָׁן:
And these are the ones who are rescued [from a transgression] by their lives, [all men having the right to kill them to save them from the transgression]: one pursuing another to kill him, [it being written in respect to (the ravishing of) a betrothed maiden (Deuteronomy 22:26): "For as a man would rise up against his neighbor and slay him, so is this thing." Slayer is hereby being likened to betrothed maiden, viz.: Just as a betrothed maiden may be saved by killing the pursuer, so a slayer may be saved (from transgression) by killing him. And betrothed maiden is derived from a verse, viz. (Ibid. 27): "The betrothed maiden cried out and no one could save her," the implication being that if one could save her, he could resort to any means (even killing, if necessary,) to do so.], (one pursuing) a male (to sodomize him [This is derived from (Ibid. 26): "And to the maiden (na'arah) you shall not do a thing." It is written "na'ar" (young man), without the heh. The same applies to all who are liable to kareth and judicial death penalties in the area of arayoth (illicit relations) — that they are rescued (from transgression) by their lives, it being written (Ibid.): "a sin of death": "sin" — kareth liability; "death" — judicial death penalty.], and (one pursuing) a betrothed maiden. But one pursuing a beast, [though this is similar to arayoth], and one who would desecrate the Sabbath or serve idolatry [though both of these deny the Deity] are not saved (from transgression) by their lives. [And, it goes without saying that the other would-be transgressors who are liable to kareth and judicial death penalties not related to arayoth, are not saved by their lives. It is not permitted to kill them at all until they commit the transgression in the presence of witnesses and become liable to judicial death penalty.]
Bartenura on Mishnah Sanhedrin
English Explanation of Mishnah Sanhedrin
But he who pursues after an animal [to have relations with it], or one who would violate the Sabbath, or commit idolatry, must not be saved [from sinning] at the cost of his life.
The final mishnah of chapter eight continues to discuss this concept of “preventive punishment” and its limited applicability in Jewish law.
The principle of preventive punishment is a dangerous principle for even a court cannot tell with certainty if someone will surely commit a crime in the future. The Rabbis recognized the danger of this principle and therefore limited its applicability. Section one teaches that one may be killed preemptively only if he was about to commit a capital crime that would violate another person. Section two lists cases in which a person may be killed preemptively, even though he is about to commit a capital crime. Since none of these sins are crimes against other people, the only way the criminal can be executed for having committed one of them is by a proper trial done in front of a court of twenty three.