For me, personally, I think the heart is Kathryn’s. The heart is a literal human heart—not the magical heart like we saw in “The Heart is a Lonely Hunter.” If the heart, is in fact, not Kathryn’s, it most certainly is someone’s heart (and I’m sure Regina has lots of people she’s wanted to kill). I don’t think it’s a magical heart seeing as how it’s not glowing.
I think it’s obvious (to me anyway) that Regina put that box there and framed Mary-Margaret with the fingerprints. I’m curious how Regina got the fingerprints to start with 😆