What you're describing is the primary concern of historiography. Everything we learn about history is conveyed to us through the eyes of someone else, and therefore it is essential to study who that person was and know their bias so that we can interpret their writings with better precision. Every event has different sides to the story, different vantage points and different agendas that are written into the context of every historical event recorded throughout history. Studying the writings within the context of the original bias helps bring the legitimacy or accuracy of such writings into focus.

For instance, the event of burning the library to the ground was a huge blow to the civilized world and some may have taken the opportunity to cast the blame on their political or idealistic rivals in order to make them out to be more cruel or unpopular.

You might be interested in this article about the library of Alexandria. It's a bit old (a little over a decade) but to the point. He points out an issue with the question you raise, mainly that we are fairly sure there were multiple libraries in Alexandria, and even without major destruction such as through fire, the papyri in the library would be unlikely to survive in usable condition for very long in a non-desert climate.

So, maybe there was a major destructive event, maybe there were many, but perhaps the loss was from the slow and steady ravages of time. We're simply not sure. Either way, it makes for an attractive story, hence the multitude of claims about how the library was destroyed.