[Vader may eventually discover that he and Grievous have more in common than either of them knew. Both manipulated. Both lost somebody very important to them. Both made more machine than flesh and used by Darth Sidious for the Sith's own ends. Much of this Grievous himself was even unaware of, at least at the present time.]

So you finally understand that the Jedi are not what they claim to be. Their agenda is only to further their own means and they do not come to those who would give them a fairer and balanced judgement on any situation. Their ways are arrogant and narrow-minded.

Perhaps you see better now why they needed to be stopped.