How did the Spanish treat the Aztecs once they were conquered?

It really depended on the person. On the one hand you had a soldier of fortune looking for gold, who couldn't care less about the Aztec as long as he got the gold. Then sometimes the missionaries would remind the soldiers of fortune to treat the natives respectfully as people and teach them. The soldiers would probably say, "Oh yeah, don't worry we'll take care of them, ha-ha-ha...", and then cut their throats as they took more gold and burned down their buildings. Then the given missionary might truly treat the Native well, but other individuals might treat them as lowly savages, not understanding them at all. Other times you would have a regular guy coming in from Spain just to make a better living, and that guy would fall in love with an Aztec girl and marry her, and probably treat her very well. But then the neighbors would look at them and not treat them well because they had a mixed marriage. There is no good answer. It really depended on the given person.