Why did Vader (Anakin) finally rebelled against the Emperor?

Why did Vader (Anakin) finally rebelled against the Emperor?

In Episode 3, he helped carry out Order 66 to kill all Jedi.
In Episode 4, he helped carry out Order 67 to destroy Alderaan.
In Episode 5, he helped carry out Order 68 to destroy rebel base on Hoth.
In Episode 6.... the Emperor was just too old and nasty for Vader to carry out Order 69.

EDIT: To satisfy grammar nazi