I think they wanted to portray the Legion as stable (so long as they had new lands to conquer and their charismatic leader doesn't die lol), but deeply evil. That trader you reference says he likes to travel through Legion lands because there are no bandits and he knows what to expect, but he is a man, and he has things the Legion wants. If you're a poor tribal boy? Straight into the meat grinder. A women? A nightmare awaits. They're a realistic evil, especially compared to the comical bad guys of the Bethesda Fallouts. Caesar is also such a jackass, he acts like he is removed from what the Legion does, that he is instead just acting out the historical moment. He thinks hes history on horseback, but hes about to die from cancer and the NCR is going to rip the Legion a new one even if he conquers Vegas.