If you think about it, what's the use of having a Demon Lord character in a story if he/she doesn't do some Demon Lord-ish things from time to time. A little random chaos serves to keep the other characters on their toes.
As for Viola, she's got something devilish about her elfin self, which is fine since it gives Noel, Chrys-san, Kumon, & Liliam chances to be helpful to Reene.