Laudator Temporis Acti

Diodorus Siculus 1.83.6-9 (tr. C.H. Oldfather):

[6] And whoever intentionally kills one of these animals is put to death, unless it be a cat or an ibis that he kills; but if he kills one of these, whether intentionally or unintentionally, he is certainly put to death, for the common people gather in crowds and deal with the perpetrator most cruelly, sometimes doing this without waiting for a trial. [7] And because of their fear of such a punishment any who have caught sight of one of these animals lying dead withdraw to a great distance and shout with lamentations and protestations that they found the animal already dead. [8] So deeply implanted also in the hearts of the common people is their superstitious regard for these animals and so unalterable are the emotions cherished by every man regarding the honour due to them that once, at the time [60 or 59 B.C.] when Ptolemy their king had not as yet been given by the Romans the appellation of "friend"​ and the people were exercising all zeal in courting the favour of the embassy from Italy which was then visiting Egypt and, in their fear, were intent upon giving no cause for complaint or war, when one of the Romans killed a cat and the multitude rushed in a crowd to his house, neither the officials sent by the king to beg the man off nor the fear of Rome which all the people felt were enough to save the man from punishment, even though his act had been an accident. [9] And this incident we relate, not from hearsay, but we saw it with our own eyes on the occasion of the visit we made to Egypt.

Anne Burton ad loc.:

Herodotus, II, 65 says that the unintentional killing of a hawk or an ibis meant death. Wiedemann, Herodots zweites Buch, p. 282, suggests that this varied from nome to nome, depending on which animals were considered sacred locally. But since the hawk and ibis (Horus and Thoth) were held to be sacred throughout most of Egypt, Herodotus is probably substantially correct. Presumably Diodorus includes the cat because he himself saw the incident he describes. This must have happened in a district where the cat was held to be particularly sacred, and it is therefore reasonable to assume that Diodorus visited Bubastis.

Alexander Meeus, "Life Portraits: Royals and People in a Globalizing World," in Katelijn Vandorpe, ed., A Companion to Greco-Roman and Late Antique Egypt (Hoboken: John Wiley & Sons, Inc., 2019), pp. 89-99 (at 94):

Since this is one of very few such personal experiences that Diodorus has included in his work, it is clear that it must have made a great impression on him. Although it has been observed that Diodorus' attitude toward Egypt was more positive than that of many of his contemporaries (Isaac 2004, p. 359; Muntz 2017, pp. 228–231), the event may have made such an impression because the animal worship appeared so bizarre to him (cf. Smelik and Hemelrijk 1984):

As for the various services which these animals require, the Egyptians not only do not try to avoid them or feel ashamed to be seen by the crowds as they perform them, but on the contrary, in the belief that they are engaged in the most serious rites of divine worship, they assume airs of importance, and wearing special insignia make the rounds of the cities and the countryside. (Diodorus Siculus 1.83.4, translation Oldfather, Loeb Classical Library)

Was this the embassy of which Cicero would have been a member, had he accepted the invitation to join (Cicero, Epistulae ad Atticum 2.5.1)? At any rate, we would love to know how the Senate reacted to the execution of a Roman ambassador over the killing of a cat, and one wonders whether Cicero was thinking — among other things — of this episode when he asked some 15 years later:

Who does not know of the custom of the Egyptians? Their minds are infected with degraded superstitions and they would sooner submit to any torment than injure an ibis or asp or cat or dog or crocodile, and even if they have unwittingly done anything of the kind there is no penalty from which they would recoil. (Cicero, Tusculanae disputationes 5.78, translation King, Loeb Classical Library)