Why did Hashem enable David to see the angel?
Radak (Shmuel II, 12:16): (a) The people should see David pray, and that 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 (verse 26).
When Yehoshua saw an angel bearing a sword, he asked "are you for us or our enemies?" (Yehoshua 5:13). Why didn't David ask?
'Rashi': Yehoshua's back was towards his enemies, and the angel was in front of him. He was unsure if the angel came to help him (for he faces Yehoshua), or to help our enemies (the angel faced also the enemies). Here, the sword was against Yerushalayim, so it was clear that he intended to strike Yisrael 1 .
Even though David requested Dever, and he presumably knew that many died already, if not for the sword, he could have thought that the angel came to stop the Dever. (PF)
Why was the angel between the ground and Shamayim?
Malbim: He distanced from the ground because he was not authorized to strike. He was not authorized to return to Shamayim, Initially his sword was against Har Tziyon. After Hashem told him "Heref Yadecha" (verse 15), he reversed it towards Yerushalayim.
