Images and texts in Spanish colonial manuscripts tell us that the Aztecs sacrificed humans. Among these sources, none has received a more iconic documentary status than the Franciscan missionary Bernardino de Sahagún’s tlamictiliztli account. This combines graphic depiction with text in Spanish and the Aztec language Nahuatl. Although Sahagún’s account is compelling and persuasive, its seeming realism dissipates when the analytical focus turns from the final product to its production. Using a new source-critical narrative approach that privileges methodological reflection, I analyze two versions of Sahagún’s composition to examine how the account was elicited, editorialized, and produced. Astonishing inconsistencies stress how Sahagún augmented a ‘sacrificial’ interpretation of Aztec violence, complicating both the testimonial value attributed to his work and an emic Aztec sense of ‘human sacrifice.’