Entropy Judge
Recurring Bouts of Misanthropy
Flandre is generally "sane" - she is uncaring of human life unless she has an ulterior motive, does what she pleases with little regard for others, and reacts violently and without real control over how much power she uses. "Madness" occurs in spikes, however, so while the idea is thoughtful, it wouldn't be of much use because it would generally read "normal."
It would be more correct to say that the Yellow-voice is a reaction to Flan's toys being broken - you can't have fun if your toys break, after all, and if the only person who can really play with you isn't around too much ....It's the reason why Flandre's toys end up broken. Plays too hard. It probably came about when Flandre got locked in a goddamn basement forever, and Headless didn't have anything to do because there weren't any people to slaughter.
Also, Flan was hardly 'locked up' - both she and Remilia knew that nothing could really stop her from getting out if she wanted to, and Flan understands why her being away from everything else is for the best. She doesn't like it, but as far as she can tell, there's not much they can do about it. It's just so much easier on everyone if she stays in her own area where she can't break something useful/valuable and make Remilia scold her.