1)
What do we learn from "Im Chatasa Mah Tif'al Bo"?
1.
Malbim: Your sins cannot change Him. They do not even anger or sadden Him. Wherever it says that Hashem is angry or sad due to people's actions, it pertains to those who receive [influence]; Hashem does not change.
2)
Why did he add "v'Rabu Pesha'echa Mah Ta'aseh Lo"?
1.
Malbim: Even if you sinned intentionally or rebelled, which is worse than Chet, and even if they are many, not do you not affect Him - you do not even blemish His honor!
