best ending theory (spoilers evidently)

The eyes of Jane Doe get their color back, so that's an improvement of her state since her arrival at the morgue. This indicates that while toturing (and killing) the father at the end she regained more "life" power than she had at the start of the film.

Her goal is to live again. But the son killed the father before the "witch" could finish him off.
That's why she didn't resurrect completely. And also that's why she killed the son too.
Revenge for killing the father before she could finish her life drain.

The father understood what had to be done. He made kind of a pact with her. He willingly accepted to endure all the torture to the end. But the terrible pain was too much for him to handle, so he asked his son to shorten the pain by killing him quickly.

If he died willingly by her will, she would be alive again. And maybe the son wouldn't have died.

This "witch" messes with the minds of people, she has control over people perceptions.
She's manipulating the livings minds (the cat too). Some kind of hallucinations (mostly audio and some visual) happen to people near the body

I'm too lazy to write other elements that point to that theory, but if you rewatch the film, you'll probably see what i mean