1)
Why did he request mercy?
1.
Radak: So I will live, to guard Your Torah.
2.
Malbim: For consolation for what he suffered, he appealed for Chesed. For this he appeals for mercy, since "Your Torah is my delight", and Torah weakens. I request to live via Your mercy, especially since "Chayim Hem l'Motze'eihem" (Mishlei 4:22).
