- Muslims killed 500 people at Alexandria for failing to pay the jizya tax.
- Muslims killed 500 people at Alexandria because they're intolerant jerks.
Sure, both sentences are true, but honestly, I don't need the morality of the event spoon fed to me. I just want to know what happened and why. I realize that some people feel slighted if the full moral dimension of the story isn't added, but I would argue that it's preferable for historians to play it as straight as possible, and let future generations draw their own conclusions.
The informational content of the two sentences is different, which partly accounts for your preference.
Here we go: “Muslims, because they were intolerant jerks, killed 500 people at Alexandria for failing to pay the jizya tax.” ;-)