I think Regina burned RH’s wife at the stake this will kind of mirror of what Rumple did to his wife since he killed her, Regina kills Marian and RH will go after her once he finds out after all so far they haven’t said her name to each other have they? After all RH said that his wife was killed for a job that he did and Regina had posters on him to0.
Note: Snow still has her fairy dust with her.
Note: Regina wanted Snow’s heart not to burn her at the stake so this Marian that is at the stake because of RH’s job not because of Snow.