capital punishment - Victim of an Accidental Killing - Source

I recall learning (many years ago) that in a case of accidental killing that qualifies the "killer" to go to an Ir Miklat, the victim in the case was someone who deserved a death penalty but the human court could not carry through with that punishment, so the "accident" was divinely arranged so that justice could be done.

Ignoring the theological implications, I am wondering if anyone knows a source for this (other than my possibly over active imagination).

Answer

You are likely thinking of Rashi to Shemot 21:13:

והא-להים אנה לידו: ולמה תצא זאת מלפניו, הוא שאמר דוד (שמואל א' כד יג) כאשר יאמר משל הקדמוני מרשעים יצא רשע, ומשל הקדמוני היא התורה, שהיא משל הקב"ה שהוא קדמונו של עולם. והיכן אמרה תורה מרשעים יצא רשע, והא-להים אנה לידו. במה הכתוב מדבר, בשני בני אדם, אחד הרג שוגג ואחד הרג מזיד, ולא היו עדים בדבר שיעידו, זה לא נהרג וזה לא גלה, והקב"ה מזמנן לפונדק אחד, זה שהרג במזיד יושב תחת הסולם, וזה שהרג שוגג עולה בסולם ונופל על זה שהרג במזיד והורגו, ועדים מעידים עליו ומחייבים אותו לגלות, נמצא זה שהרג בשוגג גולה, וזה שהרג במזיד נהרג:‏
but God brought [it] about into his hand: Now why should this go out from before Him? That is what David said, “As the proverb of the Ancient One says, ‘From the wicked comes forth wickedness’” (I Sam. 24:14). The proverb of the Ancient One is the Torah, which is the proverb of the Holy One, blessed is He, Who is the Ancient One of the world. Now where did the Torah say, “From the wicked comes forth wickedness” ? [This refers to:] “but God brought [it] about into his hand.” To what is the text referring? To two people, one who killed unintentionally and one who killed intentionally, but there were no witnesses who would testify to the matter. This one [who killed intentionally] was not executed, and that one [who killed unintentionally] was not exiled [to the refuge cities]. So the Holy One, blessed is He, brings them [both] to one inn. The one who killed intentionally sits under a ladder, and the one who killed unintentionally is ascending the ladder, and he falls on the one who had killed intentionally and kills him, and witnesses testify about him and sentence him to exile. The result is that the one who killed unintentionally is exiled, and the one who killed intentionally was killed. -[From Mechilta, Makkoth 10b] (translation from Chabad.org)