What was Slayed:  The Mayor as he had ascended to a huge worm-like demon.
His faults:  Trying to eat the humans of Sunnydale.
Who Slayed Him:  Buffy, Giles, and the graduating population of Sunnydale High.

What was Slayed: Pack of vampires
Their Faults:  Helping the mayor carry on his ascension.
Who Slayed Them:  Angel with the help of the graduating population of Sunnydale High.