/why does everyone think that when the spanish conquered the aztecs it was a bad thing/
Because killing and enslaving others is wrong, maybe?
/it was a good thing for everyone/
Tell that to the indigenous tribes of Mexico, the modern-day descendants of the Aztecs. I'm sure they can tell you how wonderful it was and it turned out well for them.
/the aztecs killed 50 people like every single day/
As opposed to the Spaniards killing thousands of people by conquest and disease?
/when christianity came it saved lots of people from being sacrificed/
Hmm, to choose between being sacrificed to the Aztec gods or being sacrificed to the Christian God. It may be just me, but I really don't see much of a difference.
/and now the mexicans are christians so they are saved/
Yeah, like those African slaves. How dare African-Americans today complain about slavery! Don't they know that if their ancestors hadn't been enslaved, shipped over to the Americas like cattle, and tortured and raped, they could never have become Christian and would have instead clung to their pagan gods? How ungrateful they are!
/and the spanish got gold/
Correction: the Spanish STOLE gold. They "got" gold at the beginning when Moctezuma offered them gold, thinking that they were gods. After that, however, the Spaniards pretty much took whatever gold they could lay their hands on, regardless of whether or not the Aztecs said it was okay.
/its a win win for everyone/
Oh, how I hope the Nahuatl tribe (descendants of the Aztecs) comes to your house and gives you a violent smackdown.
/why do some people think it was bad/
BECAUSE GENOCIDE IS A BAD THING! BECAUSE IT IS NEVER RIGHT TO KILL PEOPLE FOR GOD! Why do you insist on being so deluded and dense?