It is safe to assume that lightning bolt didn't strike and Grimhilde killed them. Then to make "doubly sure" Snow White stays dead, she cut out her heart and put it in that box. Then changed herself back.
Also: until I viewed this and your Malificent portrait, I hadn't noticed that the two most awesome of all Disney villainesses both had pet corvids! (It strikes me as fitting that Malificent has more of a wickedly sleek raven and the Wicked Queen has more of an ill-fed crow -- like one of those crows found on the battlefields pecking away at the dead.)
This one for sure is the creepiest so far with the blood dripping out of the chest. I love the look of pleasant contentment on Queen Grimhilde's face and the flowiness of her robes. She looks super regal.
I worked in the movie theaters, so I always give a customers a punchline based on each movie. For Snow White and the Huntsman, I always like to remind customers that it's basically about a vampire lover and the god of thunder.