1)
Why did he say "k'Chasdecha"?
1.
Radak: Your Chesed was with me many times. I request that You keep me alive further, until I see vengeance on my pursuers.
2)
Why did he say "v'Eshmerah Edus Picha"?
1.
Radak: In my life, I guard Your Edus. [My pursuers] transgress Torah in their lives!
2.
Malbim: Sustain me like Your Chesed, in order that I can guard Edus of Your mouth. Via guarding Pikudim, he establishes their testimony about Hashem's miracles, wonders and Hashgachah. This is according to the Torah's testimony of Yetzi'as Egypt and all the signs that He did with Yisrael in the Midbar, on which the Pikudim were established.
