If Aztecs were not conquered?

[Aegon]

What would happen if Aztec empire still existed? What would their civilization be like?

Would they have gotten over their human sacrifices and embraced Christianity voluntarily?

Or would they have remained devoted to sacrificing people and become an "Evil empire" sort of like the Soviet Union was?

I know you might be trolling, but fuck it, I'll take the bait because I want to talk about this...

I take issue with your loaded question because it assumes their society is evil and savage compared to the great, pure Christianity. The actions by conquistadors in Mesoamerica were ten times as "savage" as anything the Aztecs ever did. People have it in their heads that Aztec rulers just picked poor, innocent people off the street and ripped their hearts out and threw their bodies to the buzzards. That would have been seen as savage. Human sacrifice was a solemn and delicate process. Most sacrificed were P.O.Ws who fought nobly in war or members of their own society; in either case, it was seen as a great honor. The Aztecs kept a ritual calendar, and the calendar called for different sorts of sacrifices. Some ritual participants needed to be tall, handsome, lean, and extremely attractive in order to satisfy the prerequisites of a given ritual period and for other ritual periods they had to be fulfilled by children. The process to becoming a ritual participant was actually very demanding in many cases, requiring the participant to learn many speeches and dances before being sacrificed. It was just a natural part of their society. 

As for your overarching question, it would have happened. Around 50 percent of the Aztec population died from a smallpox outbreak in the first few years after making contact with the Spanish. They were doomed from the start.