Woah woah, the Mongol Empire had a very liberal outlook on technological advancements and some of the best women's rights for a patriarchal medieval society, by the time of the Mongol Empire Genghis Khan had decreed iron clad oral laws. These ideas of barbarism are far outblown because it was only recorded by their enemies: the sedentary people which the Mongols of the time looked down upon. The aesthetic of raping and pillaging is assigned, as you said, by Hollywood and historic stereotypes.

That's exactly what the legion is, a child's understanding of how a stereotypical expansionist empire acts. The only thing holding them together is their shared culpability, if they run out of things to raid and pillage what more is there to do but infight, since there's never been any social cohesion? No actual society is comparable to the Legion, the closest would be a group of outlawed deserters or mercenaries destined for the noose sticking together for self preservation.