I had a dream about a little girl being held in some laboratory as a guinea pig for some scientist's experiments or something. Despite having been raised from birth inside a large but sealed room with little to no human contact and being treated as a test subject rather than a person she has a very cheery personality. I think one scientist sympathized with her though by talking to her and telling her stories of the outside world, something the little girl always looked forward to hearing about. Anyway, one day the scientist in question died (Not sure if it was natural or murder at the hands of the other scientists) and when the girl heard of this she suffered a psychotic break, escape her prison, went on a homicidal rampage with some unseen power (Likely from the experimentation), kills everyone, and destroys the facility.

Kinda sounds like something fom Elfen Lied to me.