Why did he say "v'Atah"?
Rashi: After he rebuked them for sinning against himself, he rebuked them for sinning against Hashem.
What is the meaning of "v'Ishaftah"?
Rashi: I will dispute with you.
Radak: I will judge between you and Hashem. He did righteous acts with your fathers and you, and you did evil to him!