1)
Why did he say "Ki Tov Chasdecha"?
1.
Radak: Your Chesed is good. It is not withheld from me, even though I sin!
2.
Malbim: You can save me in a good way, according to Your Chesed. I.e. he can be saved via fleeing Yerushalayim, hiding in caves, or getting refuge in a fortified city, like the metaphors of the raging current washing him away, being swallowed in the depth, or the pit closing its mouth on me, respectively.
2)
Why did he say "k'Rov Rachamecha Pene Elai"?
1.
Radak: [Turn to me with Your great mercy, and] do not turn to my deeds.
2.
Malbim: Save me in a good way amidst mercy over my affliction.
