Is Diane really 'evil incarnate'? SPOILERS

Yes, she is a murderess. But unlike a true sociopath, Diane actually feels guilty about her crime -- and not just because her beloved father was killed, too. If you recall, Diane expresses grave remorse for the killing of her stepmother.

Then, later on, she makes that formal statement to her attorney in the hopes that she may still be convicted of her crime.

Again, this is not the behavior of a sociopath.