1)
Why did he say "ha'Lo Anochi Tzivisi"?
1.
Radak: Sometimes "ha'Lo" is to make zealous and invigorate, e.g. "ha'Lo Shelachticha" (Shoftim 6:14), "ha'Lo Ki Meshachacha" (Shmuel I 10:1).
2.
Malbim: You will be not be punished, for I commanded you. 1
1
Perhaps he means that David will not punish them. Hashem will punish them, for there is no Shelichus for an Aveirah! (PF)
