Normally in modern fantasy writings, Female forces are often the "benevolent", orderly ones while Male forces are the more destructive, emotive ones. Also, Female demons rarely get representation in modern fantasy unless they have to do with lust/hypersexuality.
While Skin lore has nothing to do with the game lore, Riven, even though she willingly killed plenty, at the end she came to question her Noxian values of "strength above all" due to what she witnessed: innocents being killed without warning, she may still hold her values of strength makes right, but not exactly the unfettered "injustices" of war. On the other hand, Yasuo has also killed Noxians. He may be innocent of the murder he's accused of, and he may feel guilty about abandoning his post, his reason for killing his countrymen is still in a way selfish: he wants to clear his name of any guilt.