Not too different from what some ancient civilizations would have done it, albeit in a less elaborate way.
Mind you, there's accounts of vikings raping enemy leaders to "destroy their manlyness", and there's sufficient evidence that the vikings also had many interesting ritualistic ways of killing people, including the notorious "blood eagle". We dont even have to go that far either, the ottoman empire was know for taking their enemies' wives and daughters and making personal harems, ensuring them an entire life of sexual slavery. Roman generals were also known for taking "catamites" as well, which are basically nubile boys that served as sex slaves. Nero had a catamite that he dressed in his wife's clothes after she died, and called him by her name as well *shudders* :v
Then again nero was fuck-off tier insane, so.
Anyway, my point is, everything F:NV's legion does in game, ancient humans (and even current humans, IE ISIS) have done. Yes, you should totally smash them to bloody chunks, but they're in no way cartoonish or unrealistic, unless you want to consider ancient history unrealistic and cartoonish.
The only thing that makes the legion more reprovable then any of this is that the legion came up at a time in which other civilized entities that carried over the pre-nuclear world's morals and ethics existed, meaning they could've known better and learned to become more civilized if they/caesar wanted to. Like I said, F:NV's legion is just the roman empire with the good and empire-y bits taken out.