Although he is, for the most part, an evil being I like to think that there's a lot more to Nocturne than that. Without darkness there would be no light, et cetera. Some form of balance is needed for Nocturne to exist--at least that's what I believe. To destroy someone mentally you really have to know the full spectrum of emotions to get into someone's head. Sure, you can scare someone to death, but to really screw with someone on a subconscious level you need at least an understanding of light and love as well.
I have no idea of Nocturne and Riven could work out, but that Nocturne and Lux fic certainly did. I think what really matters is how you spin it. I've always imagined that Nocturne might have a fair share of psychopathic characteristics. If he is the nightmare of a summoner or champ, then he might exhibit some human like characteristics beyond fear, despair, blood lust, etc.
I dunno, I may sound totally off my rocker, but I think you could do a lot with this character.