Why did Hashem enable people to see the angel?
Radak: (a) They should see David pray, and Hashem accepts his Tefilah there and the plague stops. (b) It will be a sign to David that the Beis ha'Mikdash and a Beis Tefilah will be where Hashem accepted his Tefilah [and stopped the angel]. Gad told him that the Mizbe'ach will be there, and fire descended from Shamayim there to consume David's Olah (Divrei ha'Yamim I, 21:26).
Why does it say "va'Yishlach Yado ha'Mal'ach"? It should say va'Yishlach ha'Mal'ach!
Malbim: Hashem sent Yado, i.e. the angel. It says in Divrei ha'Yamim I, 21:15 "va'Yishlach ha'Elokim Mal'ach." The angel that kills via Dever is called Yad Hashem - "Yad Hashem Hoyah" (Shemos 9:3). It says in the Hagadah that "uv'Yad Chazakah" (Devarim 4:34) refers to Dever.
Until where did the angel strike?
Malbim: It was until Ir Tziyon, i.e. Ir David. The granary was between Tziyon and Yerushalayim.
Why does it say "El ha'Ra'ah"?
Radak: This is like "Al ha'Ra'ah." There are many verses like this.
Why did He say "Rav Atah Heref Yadecha"?
Radak: The plague suffices; do not destroy Yerushalayim.
Radak citing Brachos 62b: Hashem told the angel 'take their Rav.' At this time Avishai died; he was equal to the majority (Rov) of the Sanhedrin.
Malbim: If not, the angel would have killed David, for 'when the destroyer has permission [to kill], he does not distinguish between Tzadik and Rasha' (Bava Kama 60a). Hashem retracted due to the merit of David and of the place. 1
Malbim citing Brachos 62b: "Uv'Hashchis Ra'ah Hashem va'Yinachem" (Divrei ha'Yamim I, 21:15) -- what did He see? He saw the ashes of [the ram offered in place of] Yitzchak. Some say, He saw Kesef ha'Kipurim, i.e. the Shekalim that Bnei Yisrael will give every year for [Korbanos brought in] this place atone for (exempt from) the plague.
What is the meaning of "Im Goren"?
Radak: It is near the granary.
Why is it written ha'Avarnah, and we pronounce it ha'Aravnah?
Radak: The letters are reversed. Many words are like this.
Why does the name ha'Aravnah have the prefix Hei?
Radak #1: This is unusual.
Radak #2: Perhaps it is an adjective.
Why is he called Yevusi?
Rashi: He was the officer of Metzudas Tziyon, which is called Yevusi.
