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The link from Wikipedia gives good detail about the theories involving the library's destruction involving the Romans or Christians or Muslims. "The famous burning of the Library of Alexandria, including the incalculable loss of ancient works, has become a symbol of the irretrievable loss of public knowledge. Although there is a mythology of "the burning of the Library at Alexandria", the library may have suffered several fires or acts of destruction of varying degrees over many years. Ancient and modern sources identify several possible occasions for the partial or complete destruction of the Library of Alexandria.[24] During Caesar's Civil War, Julius Caesar was besieged at Alexandria in 48 BC. Many ancient sources describe Caesar setting fire to his own ships[25][26] and state that this fire spread to the library, destroying it.[26] [W]hen the enemy endeavored to cut off his communication by sea, he was forced to divert that danger by setting fire to his own ships, which, after burning the docks, thence spread on and destroyed the great library. — Plutarch, Life of Caesar[27] Bolstering this claim, in the 4th century both the pagan historian Ammianus Marcellinus[28] and the Christian historian Orosius[29] wrote that the Bibliotheca Alexandrina had been destroyed by Caesar's fire. However, Florus and Lucan claim that the flames Caesar set burned only the fleet and some "houses near the sea".[30] The library seems to have continued in existence to some degree until its contents were largely lost during the taking of the city by the Emperor Aurelian (AD 270–275), who was suppressing a revolt by Queen Zenobia of Palmyra.[31] During the course of the fighting, the areas of the city in which the main library was located were damaged.[15] Some sources claim that the smaller library located at the Serapeum survived,[32] though Ammianus Marcellinus wrote of the library in the Serapeum temple as a thing of the past, destroyed when Caesar sacked Alexandria.[33] 5th century scroll which illustrates the destruction of the Serapeum by Theophilus Paganism was made illegal by an edict of the Emperor Theodosius I in AD 391. The temples of Alexandria were closed by Patriarch Theophilus of Alexandria in AD 391.[32] The historian Socrates of Constantinople describes that all pagan temples in Alexandria were destroyed, including the Serapeum.[34] Since the Serapeum had at one time housed a part of the Great Library, some scholars believe that the remains of the Library of Alexandria were destroyed at this time.[32][35] However, it is not known how many, if any, books were contained in it at the time of destruction, and contemporary scholars do not mention the library directly.[36][37] In AD 642, Alexandria was captured by the Muslim army of 'Amr ibn al-'As. Several later Arabic sources describe the library's destruction by the order of Caliph Omar.[38][39] Bar-Hebraeus, writing in the 13th century, quotes Omar as saying to Yaḥyā al-Naḥwī: "If those books are in agreement with the Quran, we have no need of them; and if these are opposed to the Quran, destroy them."[40] Later scholars are skeptical of these stories, given the range of time that had passed before they were written down and the political motivations of the various writers.[41][42][43][44][45] https://en.wikipedia.org/wiki/Library_of_Alexandria

"Actually it depends on your source and no one really knows. Plutarch blamed Julius Caesar. Edward Gibbons (anti-Christian atheist) blamed Christians as he was wont to do. Bishop Gregory Bar Hebræus (anti-Muslim) blamed the Muslims."

And still another historical opinion (and I think the current one) is that no one ruler or group torched the Alexandrian Library but, rather, after the Muslim takeover it just become mostly neglected and forgotten. The "library" was actually housed in a number of buildings, several of which were burned down (some by accident, some deliberately), but again, the library as a whole seems to simply have sunk into ruin.