Riven is a 'traditional Noxian', who firmly believes in 'might makes right', generally speaking. However, the duty of the superior is to reform and uplift the weak, to make them strong, such that they can be fought to create an even stronger person afterword. Slaughtering the weak is against this ethos, and there is probably some contention on using 'allied states' in war because that is not Noxus' own strength being used. I imagine the latter point might be a good philosophical argument for them.
Thus, it would make sense for Riven to be against the Demacians, especially if she opposes Demacian values - where they seem your more typical Germanic/English monarchy rule and all that entails for the citizenry. It would be more accurate to say Riven 'hates' Demacia and its way of life, not the people. The people are just the vessels that can be changed, but the ideology is what corrupts them. The same brush can be used for Ionia, really.
The core issue is that Riven isn't portrayed as a traditional Noxian, or a radical/believer of any kind. She's stuck in this pity-pot mode and 16 year old anime girl angst that buries most of the intrigue her character is based on.