Who kills Lord Siward's son?

In Act V Scene 7, Macbeth himself kills young siward. When Macbeth identifies himself to Siward. he anticipates fear. He is more than a little surprised when he encounters vehement loathing instead; Line 4:

YOUNG SIWARD: What is thy name?                                      MACBETH:                                  Thou'lt be afraid to hear it.

YOUNG SIWARD

No, though thou call'st thyself a hotter name                       Than any in hell.

They argue alittle bit about whether Macbeth should be hated or feared. They then fight and young Siward if killed. Macbeth then gloats the Witches prophecy that none born of woman shall kill him in Lines 11-13:

MACBETH                                    Thou art born of woman.  But swords I smile at, weapons I laugh to scorn,                   Brandished by man that's of woman born.