Original: The king comes back in at this moment and thinks Haman is assaulting the queen; this makes him angrier than before and he orders Haman hanged on the gallows that Haman had prepared for Mordechai.
Simplified: The king, furious, storms out, and Haman asks Esther for mercy. When the king comes back in, he mistakes Haman for 'molest' ing his Queen, and hangs him on the gallows that Haman had prepared for Mordecai.