Chances are you come from a culture that did more or less the same.
Slavery? Genocide? Plunder? Rape? Religious violence? Slaughter of peoples they considered inferior? The "civilized" Christian cultures of the time did everything the Aztecs did, minus the human sacrifice.
One is just more alien to Western culture than the other, so it seems more viscerally evil, and their means differ but their nature is the same.
The crusades were armies raised in response to the Muslim invasions of Christian countries. Motivations varied between religious (defense of the Christians being oppressed) to political (I want a kingdom to rule over) [1].
[1]: https://history.stackexchange.com/questions/7915/are-there-a...