There are societies who don't seem to have practiced sacrifice or at the very least it was limited and most sacrifice was autosacrifice. For the Late Formative/Classic period Teuchitlan Tradition in West Mexico there seems to be little to no evidence of sacrifice. The Post Classic Caxcanes in West Mexico also seem to practice little to no sacrifice. A colleague who has worked in the area for several years joked that they, the Caxcanes, must have been atheist since they do not seem to build public religious architecture and do not depict deities in any way. The Teuchitlan Tradition did have public architecture, a formation called a guachimonton, but the exact use is still being figured out. As it stands it looks to be equal parts ceremonial and domestic/location of frequent ritual feasting.

I do not what you mean by medieval Mesoamerica. The term Mesoamerica refers to a geographic area where a great deal of diversity existed throughout time and space. As far as your question is concern you should place the idea of sacrifice in the context of a particular culture and at a particular time. For example, the Mayas of the early Classic period were less concern with the sacrifice of enemies, as they emphasized the auto sacrifice (blood letting) of the Ku’hul Ahaus, the holy kings. However, by the late Classic period, as the number of political players who wanted to partake in a political process so centered in the hands of few increased, so did interpolity and intrapolity violence and the imagery related to the sacrifice of enemies. A similar shift seems to occur in the various cultures of central Mexico during the Postclassic period for whom sacrifice was not a big part, or at least as far as we can tell. This changed once the Aztecs started their imperial expansion using sacrifice as a method of coercion and fear.

The most famous individual who was documented as opposing human sacrifice was culture hero Ce Acatl Topiltzin, a Toltec priest and ruler who is thought to have lived around the 10 century in Tula, central Mexico. Topiltzin Quetzalcoatl attempt to end the practice of human sacrifice in Tollan but enthnohistorical evidence suggests that a conflict broke out between him and another faction with in Tula, ultimately leading to his expulsion from the city.

That said, human sacrifice among Mesoamerican peoples should not be characterized as an "atrocity". Within the moral context of Mesoamerica, human sacrifice was an accepted practice that was thought to be necessary for the safety and stability of all. It was not done out of malice or a contempt for people nor a general disregard for human well being. Every society has practices which, to outside observes, can be seen as atrocities.