Fray Bernardino de Sahag$$n$,$ who interviewed members of the Mexica people decades after the Spanish attack believed that

the Spanish attacked the Mexica in self$-$defense

the Spanish leader was greedy and wanted to steal the jewelry of the dancers in a non$-$violent way, but things got out$-$of$-$hand and fighting broke out.

while watching the festival, some of the Mexica dancers attacked the Spanish, who responded with violence.

the Spanish slaughter of the Mexica was premediated and intentionally savage