1)
Why did he say "k'Tzidko"?
1.
Rashi: When he justifies the final verdict, to judge Resha'im according to their evil.
2.
Radak: It is according to the righteousness that He did with me.
2)
Why will he thank and sing?
1.
Malbim: He thanks for the good that he received, and sings to tell His wonders, Hashgachah and mighty acts.
3)
Why did he say "Hashem Elyon"?
1.
Radak: He is above everything, and rules over all, and does to the creations like He desires.