There were no international laws forbidding Spain from conquering the Aztec land. Spain thought they could do anything they want and everybody was doing it to.
Aztecs thought Spaniards were good people so they let them into their city. Aztecs were going to give them everything like gold, goods and shelter. They thought Cortes was a god. Spain took what they were given.
Aztecs were barbarians and savages. Christianity was civilized. Spain wanted to give