I think that Woland's group is more mischievous than evil, though Bulgakov has chosen to clearly portray both traits.  When the old man is talking with Woland, there is an obvious maliciousness that he is treated with--taking into account the multiple stool instances, the wine, the hat/kitten, the rude inquiry into his personal finances, the revealing of his fate, etc.  In addition the hurling of the chicken at Berlioz's uncle, though awesome, was obviously quite cruel not even taking into consideration his treatment of his belongings and his suitcase.  Thinking about it I'd like to change my beginning thought from "more mischievous than evil" to "mischievous in a strongly cruel manner."