This could go into stuff I learned during Quarantine. Why the Aztecs believed that sacrifice was necessary to make sure the Sun rose was that they believed that in animals and humans there was a spark of the Sun's energy and that's why living animals (mammals and birds at least) feel warm. That energy was held in the heart and one reason why the heart kept on beating even after animal died. If they didn't release that energy the Sun would eventually run out of energy. During sacrifice not only would they take the heart out they would then burn it while still beating so the energy would be released.