Final answer:
Sir Bedivere betrayed King Arthur in "The Day of Destiny" by not following the King's orders to throw Excalibur into the lake, which resulted in King Arthur's death.
Step-by-step explanation:
In the Arthurian legend, Sir Bedivere betrayed King Arthur in "The Day of Destiny" by not following the King's orders to throw his magical sword Excalibur into the lake. Instead, Bedivere hid the sword twice, hoping to keep it for himself. He only threw the sword into the lake on his third attempt, after King Arthur strongly insisted. This act of betrayal ultimately led to King Arthur's death.