Yeah...I would think one impulse would cancel out the other. Still she could indulge the more negative impulse and give Felicia a hard time only to turn around and help once she thinks Felicia has suffered enough. As for the length between following both impulses...well that's all gonna depend on Millie. Personally I don't think I'd be able to be mean to Felicia for more than oh say a couple of days.
Ah looks like Millie's forgetting a third option that this startrek Quote makes quite clear. "When the road ahead of you splits in two. Take the third path." In other words do something else. How about kick back and do nothing. Feel a bit sad for her but snicker when she's not looking. Hehehe. That's what I'd do. Probably.