In "The Odyssey", why does Homer use the similes of the fly and the cows, then the vulture, in his description of the battle?Book 22

In Book XXII, Odysseus and Telemachus begin to slaughter the suitors of Penelope.  The goddess Athene causes the suitors to become fearful and confused, running to the other end of the court "like a herd of cattle maddened by the gadfly in the early summer when the days are at their longest." 

Odysseus and his men attack the sutors as vultures do the smaller birds.  The sens here is of helplessness to do anything to protect themselves.  They are at Odysseus' mercy as a slaughters them in his righteous rage.  Though some beg for mercy, Odysseus has none.  One by one, they all suffer the same fate.