The Tlaxcalans allied with the Spanish against the Aztecs because they'd gotten tired of the Aztecs dragging off their people and ripping their hearts out.
They had excellent reasons for hating the Aztec, however the conquistadors were not much better.
Spain and france were two countries that would sell their allies down the river as soon as there was a slight advantage in doing so. Sometimes they did it when there was really no advantage. Strange bunch.