Edward is in shock at what he sees. Before both him and Mary is the figure of a woman being burned on a stake while being surrounded by a sphere of fire. No matter how much he can try to deny it, he knows what he sees. Mary runs slightly faster than Edward because she's not injured.
Both have to stop when the heat becomes too great. The girl in the flames manages to have enough control over herself to stare at Edward. Not in a happy way, but one born of rage. This most likely is the Goode girl Edward betrayed and left to die. No wonder he's scared.
Yep, it is Susannah in the fire! When Edward starts to cry out, the vision of the girl he left to die goes away. All that is left is the pathetic sounds he's making.