Tlot-tlot, in the frosty silence! Tlot-tlot, in the echoing night! Nearer he came and nearer! Her face was like a light!
The speaker really draws out the highwayman's approach. We hear the sound of the hooves again and again, the only sound in the cold night.
He comes closer and closer, and Bess's face shines in the darkness. We're not totally sure why it's shining. Scared? Sweaty?
Her eyes grew wide for a moment; she drew one last deep breath, Then her finger moved in the moonlight, Her musket shattered the moonlight, Shattered her breast in the moonlight and warned him--with her death.
Now Bess makes her move. She opens her eyes, takes a breath, pulls the trigger, and shoots herself in the chest.
It's a desperate move, but she did it for love, to warn the highwayman about the waiting soldiers. She gave her life to try to save him.