Given the following context:

In a prologue, the witch Mother Malkin is imprisoned in an underground chamber by Gregory, the last of a knightly order known as the Falcons, who have long defended mankind against supernatural threats. Several decades later, Gregory now works as a "Spook," a roving witch hunter.
The rise of the centennial blood moon allows Malkin to regain much of her power and break free from her prison. In a confrontation with the aged Gregory, she kills his apprentice, William Bradley, before escaping. Malkin returns to her dilapidated mountain fortress, restoring it, and her sister, Bony Lizzie, to their former condition. Gregory vows revenge.
Gregory then seeks out Tom Ward, the seventh son of a seventh son, on a local farmstead, and recruits him as his newest apprentice. Before Tom leaves, his mother gives him her pendant as a talisman. On their way to Gregory's home, Tom sees a girl about to be burned by a mob as a witch. Recognizing her from clairvoyant visions he has had, he takes her from the mob and releases her. The girl, Alice, warns him not to let Gregory know about her. Alice is revealed to be Lizzie's daughter, who spies on Gregory for her. Malkin begins gathering her army to conquer mankind.

answer the following question:

Who kills the apprentice of the aged knight? 

Malkin