What is "b'Dami"?
Radak: It is my Nefesh; the blood is the Nefesh, i.e. natural life. The intellect Nefesh saw it, and said... (refer to 30:10:2:1)
Malbim: My silence and cessation of movement [after death].
What is the meaning of "Mah Betza b'Dami b'Ridti El Shachas"?
Radak: If at the end of my life I will descend to Gehinom, why do I have life? Why did You bring me to this world, if not to acquire the world to come 1 ?
Malbim: You have benefit from me in my life, that (a) I thank and praise You, and (b) I inform people of the Emes of Kevod Hashem and His Hashgachah and wonders.
This implies that one who descends to Gehinom will not get the world to come. Are not most people punished for a period in Gehinom, and then they go to the world to come? (PF)
Why did he say "ha'Yodcha Afar"?
Radak: Sins do not leave after them anything alive; everything descends to earth - "v'Yashov he'Afar Al ha'Aretz keshe'Hayah veha'Ru'ach Tashuv El ha'Elokim" (Koheles 12:7). Earth will not thank You or declare Your Emes, rather, the Ru'ach, if it merits to return to Hashem! If You pardon me, I will thank You in life and in death!
