Re: how the Legion treats women, you can actually explain that by their warlike nature and imperialistic plans. Turning women into nothing but birthing machines ensures higher rate of birth. It is a known phenomenon in the present world that the best way to control population explosion is to educate women. Most women don't want to pump out ten babies or more (consider child mortality here; if a third world woman has nine kids, the chances are she has given birth twelve times or more). If they have the means to make their own living and the knowledge to resist tradition, they won't do it. Strip them of their freedom through culture, tradition, religion or outright slavery and you remove that "problem". So yeah, while disgustingly immoral, I could see why the misogynistic slavery would be a thing.
I still wouldn't see the Legion killing kids though; better kidnap and indoctrinate them if young enough, enslave those who aren't. So yeah, that goes into cartoonish evil territory. Same with killing women when they could be used for breeding.
Many atrocities, however, can be explained by powertripping. When you give a bunch of thugs (or brainwashed militants) unrestrained run of weaker people, bad things will happen. Like someone already commented, ISIS is a fine example of that - or stuff like the Japanese did in China.