The slaying of the Jedi so easily by Palpatine was meant to be symbolic, in my opinion. The Jedi had become weak in the force(as Mace Windu had said in Episode II). Complacency, arrogance. These had diminished the Jedi Order's effectiveness. While the Sith had been cultivating their strength.
When Palpatine took the Jedi down so easily, it was meant to show the end result of the corruption of the order constrasted with the emerging power of the Sith. When the Sith finally revelaed themselves to the order in full strength, the Jedi were not prepared. That was always my take on it.