Chances are they were killed by Cortez’s Indian allies. Cortez marched on Mexico City with a few hundred European soldiers but over ten thousand Indians. The whole raiding their neighbors for human sacrifices thing didn’t make the Aztecs very popular.
Oops, I should post faster. I essentially repeated your post.
"The epidemic of cocoliztli from 1545 to 1548 killed an estimated 5 million to 15 million people, or up to 80% of the native population of Mexico (Figure 1). In absolute and relative terms the 1545 epidemic was one of the worst demographic catastrophes in human history, approaching even the Black Death of bubonic plague, which killed approximately 25 million in western Europe from 1347 to 1351 or about 50% of the regional population.
"The cocoliztli epidemic from 1576 to 1578 cocoliztli epidemic killed an additional 2 to 2.5 million people, or about 50% of the remaining native population.
G-D bless La Malinche and Don Cortez.