Regarding evil, it says "Chashavti" (verse 8). Why does it say regarding good "Amarti"?
Radak (9): This hints that a Nevu'ah of evil is not sure to take effect, but a Nevu'ah of good is sure to take effect.
Malbim: Regarding evil, Hashem retracts also from the intent, and the evil is automatically Batel. Regarding good, He retracts only from the speech, but not from the intent, for the intent remains also now. Also breakage is in order to fix and benefit. Refer to 18:10:2:2.
Why does it say "Leheitiv Oso"?
Radak: This is like Leheitiv Lo. Similarly, "Es Mi Higadta Milin" (Iyov 26:4) is like l'Mi.
Malbim: This hints that the purpose of retracting is to improve the person, and return him to be good.
