Is the Mexica (Aztec) practice of human sacrifice reasonably justified by their religious beliefs? Why or why not?

It was reasonably justified because they believed that it worked and as they were religious they believed that what they did actually worked and appeased the gods. The human sacrifices actually did it voluntarily, although probably not all, since they believed that it was their duty. This was not unlike numerous other places throughout history where human sacrifice to gods was a common thing and done all the time. They didn't know any better and actually believed it to be true.