Where is the Halacha about a single available Cohen committing a crime?

Question

There is a Halacha about one unique Cohen remaining in Eretz Yisrael and the consequence were he to commit a crime.

I am seeking the original source as I only have a translation:

If there was only one Cohen in the country, and if that unique Cohen would have committed a crime, you would deprive the Temple of his servant rather than let a crime go unpunished.

It may relate to Exodus 21:12, where it states, "Anyone who strikes a person with a fatal blow is to be put to death." This principle of retributive justice applies to all individuals, regardless of their priestly status.

Answer 1

It’s in the second Tosafos on Yevamos 7a:

שנאמר מעם מזבחי - ואפי' אם אין שם כהן אחר ראוי לעבודה שהעבודה נדחית וא"ת ודלמא התם לאו משום דרציחה דחיא עבודה אלא משום דגברא הוא דלא חזי כדאמרי' (ברכות דף לב:) כהן שהרג נפש לא ישא את כפיו שנאמר ידיכם דמים מלאו וכ"ש עבודה וי"ל דע"כ חזי דאין מחלל עבודה דמעם מזבחי ולא מעל מזבחי ולא ישא את כפיו חומרא בעלמא הוא ועוד דוקא לא ישא את כפיו לפי שהרגו בידו ואין קטיגור נעשה סניגור וכדכתיב בההוא קרא ובפרישכם כפיכם

As it says: “Away from my altar [shall you take him to die]” (Exodus 21:14) - This is even if there is no other Kohen qualified to do the Avodah, and the Avodah will be pushed off. Perhaps this is not because execution overrides Avodah, rather, because the Kohen is not Kosher for the Avodah! We say (Berachos 32b) that if a Kohen murdered, he may not give Birkas Kohanim, for it says "Yedeichem Damim Male'u." All the more so he may not do Avodah! Answer: You are forced to say that he is qualified, for he does not disqualify the Avodah, for me'Im Mizbechi (if a Kohen must be executed, even if he is next to the Mizbe'ach and about to do Avodah, we immediately take him for execution), but not me'Al Mizbechi. (If he is on the Mizbe'ach, he finishes the Avodah, and then we execute him.) [...]