What is the grammatical form "u'Moseseni"?
Malbim: It is a four letter root, to finish death.
What is "ha'Shavatz"?
Rashi citing Targum Yonasan: It is trembling [Malbim - of death].
Rashi citing Yalkut Shimoni 141: It was due to the sin of killing the Kohanim, about whom it says [that they 1 wear] "Kesones Tashbetz" (Shemos 28:4).
Radak #1: It is the name of an illness. Perhaps it occurs due to a blow from a sword.
Radak #2: Some say that the soldiers on horses wear Tashbetz. It says "Ba'alei ha'Parashim Hidbikuhu."
Rambam (Hilchos Klei ha'Mikdash 8:16): Also a Kohen Hedyot's Kesones is "Tashbetz" (even though that is written regarding the Kohen Gadol). Ma'asai l'Melech - he learned from this Drashah, which implies that all Kohanim wear it!
Why did he say "Kol Od Nafshi Bi"?
Rashi: Kill me quickly - it is better that you kill me, and these [Pelishtim] should not kill me and disgrace me.
Radak: I am in such pain due to the blow, that I am still alive - therefore I want you to bring my death close.
Malbim: My soul is still tied to me, and it is hard for it to leave. Bringing my death close is not murder - I am already a dead man! You merely shorten my afflictions.