I mean if she would be a Demon, she would kill him on sight ... no?
Succubi used to be classified as tanar'ri (demons) prior to 4e, for example, and they aren't "mindless killing machines". The core idea was just that devils represent a hierarchic, disciplined, and purpose-driven evil, while demons are all about chaos, discord, and individuality. Hence the difference between an imp (devil) and a quazit (demon), for instance.
I dunno why they decided to shuffle the succubi around and throw them in the same-ish camp as erynies, who're supposed to be their ideological enemies.