Final answer:
Ammianus Marcellinus's harsh portrayal of the Huns was likely influenced by cultural bias, the terror and destruction resulting from Hunnic invasions, and political motives to rally Roman defenses. These descriptions also offered a mirror for Roman society itself.
Step-by-step explanation:
The motives behind Ammianus Marcellinus's harsh description of the Huns can be attributed to several factors. First, there may have been an element of cultural bias, as Roman writers often portrayed 'barbarian' groups in a negative light to emphasize their own civilization's superiority. Second, the Hunnic invasions and raids caused extensive destruction and fear, prompting writers like Ammianus to emphasize their ferocity and threat to the Roman world. Third, there was likely a political motive, in which depicting the Huns as savage enemies served to rally support for the Roman military efforts against them and justify the defense expenditures.
Descriptions of foreign groups in historical texts often served as a commentary on the narrator's own society, offering a contrast that highlighted Roman virtues or, conversely, internal failings. Thus, Ammianus's portrayals could also reflect on the Roman military and political situation of his time.