And the Spanish knew this? All about germs and diseases? No, they did not.

The Spanish were there to trade. By force yes, but whatever worked would do too.
Later they 'moved in' and the Jesuits did their poisonous thing, but the first waves of Conquistadors were not there to colonize. They all went home eventually.

The Aztecs were so terrible the other tribes risked their very existence in the hope that the Spanish could save them.