This is a famous one, and we should not go past today’s daf without mentioning it. The Mishnah on Sanhedrin 37 discusses how to cast fear of the repercussions of false testimony into the witnesses. It reads, in part:
הֲווֹ יוֹדְעִין, שֶׁלֹּא כְּדִינֵי מָמוֹנוֹת דִּינֵי נְפָשׁוֹת. דִּינֵי מָמוֹנוֹת – אָדָם נוֹתֵן מָמוֹן וּמִתְכַּפֵּר לוֹ. דִּינֵי נְפָשׁוֹת – דָּמוֹ וְדַם זַרְעִיּוֹתָיו תְּלוּיִין בּוֹ עַד סוֹף הָעוֹלָם.
The court tells them: You should know that cases of capital law are not like cases of monetary law. In cases of monetary law, a person who testifies falsely, causing money to be given to the wrong party, can give the money to the proper owner and his sin is atoned for. In cases of capital law, if one testifies falsely, the blood of the accused and the blood of his offspring that he did not merit to produce are ascribed to the witness’s testimony until eternity.
שֶׁכֵּן מָצִינוּ בְּקַיִן שֶׁהָרַג אֶת אָחִיו, שֶׁנֶּאֱמַר: ״דְּמֵי אָחִיךָ צֹעֲקִים״. אֵינוֹ אוֹמֵר ״דַּם אָחִיךָ״ אֶלָּא ״דְּמֵי אָחִיךָ״ – דָּמוֹ וְדַם זַרְעִיּוֹתָיו. דָּבָר אַחֵר: ״דְּמֵי אָחִיךָ״ – שֶׁהָיָה דָּמוֹ מוּשְׁלָךְ עַל הָעֵצִים וְעַל הָאֲבָנִים.
The proof for this is as we found with Cain, who killed his brother, as it is stated concerning him: “The voice of your brother’s blood [demei] cries out to Me from the ground” (Genesis 4:10). The verse does not state: Your brother’s blood [dam], in the singular, but rather: “Your brother’s blood [demei],” in the plural. This serves to teach that the loss of both his brother’s blood and the blood of his brother’s offspring are ascribed to Cain. The mishna notes: Alternatively, the phrase “your brother’s blood [demei],” written in the plural, teaches that that his blood was not gathered in one place but was splattered on the trees and on the stones.
לְפִיכָךְ נִבְרָא אָדָם יְחִידִי, לְלַמֶּדְךָ שֶׁכׇּל הַמְאַבֵּד נֶפֶשׁ אַחַת מִיִּשְׂרָאֵל, מַעֲלֶה עָלָיו הַכָּתוּב כְּאִילּוּ אִיבֵּד עוֹלָם מָלֵא. וְכׇל הַמְקַיֵּים נֶפֶשׁ אַחַת מִיִּשְׂרָאֵל, מַעֲלֶה עָלָיו הַכָּתוּב כְּאִילּוּ קִיֵּים עוֹלָם מָלֵא.
The court tells the witnesses: Therefore, Adam the first man was created alone, to teach you that with regard to anyone who destroys one soul from the Jewish people, i.e., kills one Jew, the verse ascribes him blame as if he destroyed an entire world, as Adam was one person, from whom the population of an entire world came forth. And conversely, anyone who sustains one soul from the Jewish people, the verse ascribes him credit as if he sustained an entire world.
Presumably, the דָּבָר אַחֵר alternative explanation of the plural demei of Hevel is not relevant to scaring the witnesses, and is not something the court would actually tell them. Rather, the Mishnah is quoting from some other existing midrashic source, and only using the first explanation, that it refers to him and any potential descendants.
If the prooftext is from Hevel, or from Adam who was created alone, it does not make much sense for it to be שֶׁכׇּל הַמְאַבֵּד נֶפֶשׁ אַחַת מִיִּשְׂרָאֵל and הַמְקַיֵּים נֶפֶשׁ אַחַת מִיִּשְׂרָאֵל, referring to a Jewish life in particular. This should be true of any human life. Indeed, Rashi writes: לפיכך נברא אדם יחידי - כל זה אומר להם לפיכך נברא אדם יחידי להראותך שמאדם א' נברא מלואו של עולם:
Let us look at printings and manuscripts. The first five indeed have “Jewish” in both places:
but the first three are printings (Vilna, Venice, Barco); Yad HaRav Herzog, as we recently discussed, is a 16th century manuscript and generally follows the text (and errors) of the contemporary printings. Florence is indeed earlier, mid-13th century.
There is also this fragment, which have מישראל in both places.
Here are the ones that don’t have מישראל. Munich 95 and CUL: T-S F 2(1).26.
and
Finally, Reuchlin 2 splits them, as follows, so the destroying a life omits it and sustaining a life includes it:
I don’t think that this shift was due to frumness or nationalism, even if such biases may have subtly influenced a scribe to interpret some ambiguous and obscure writing in an incorrect way.
Rather, Munich 95 is the key, and the awkward phrasing of ma’aleh ke`ilu or ma’aleh ‘alav ke`ilu, which other manuscripts just obliterate in favor of ke`ilu.
Look how that ayin tucking into the bottom of the lamed looks like a shin or sin; and how the lamed without the roof looks like it could be a resh. And how the left tip of the mem could be interpreted as a yud. And how often we have shorthand apostrophe strokes, even in that same image, like in the very word מעל for מעלה and elsewhere. Especially since many other manuscripts don’t have that phrase, someone could have looked at Munich or another similar manuscript and thought it was shorthand for mi-yisrael.
Of course, in a capital case before Sanhedrin we are talking about a Jewish life, but that doesn’t mean that it was true or explicit in the source derasha, or in what they instructed the witnesses.
Argument for your interpretation: the Yerushalmi's version doesn't have the word ישראל.
Argument against: The Rambam does have it (I checked Rav Sheilat's edition and he doesn't mention any MSS disagreeing). This means the reading is a lot older than MS Munich.
In general, I think this is a case where you have to look in citations in Rishonim and Geonim to judge which girsa is best. MSS gives you the range of possible readings, but don't really tell you which one is more original.