Musar sur Les Nombres 17:13

We can now answer another difficulty. When the Jewish people said to Moses and Aaron: "You have killed the people of G–d,” the Torah reports in 17,11, that Moses asked Aaron to "take the censer, place fire and incense on it, walk quickly amongst the people, כי יצא הקצף החל הנגף for the wrath has gone forth, the plague has started" (17,11). Our sages have said that when Moses was in heaven every angel revealed some secret to him. The contribution of the angel of death was that incense can prevail against the spread of the plague. This statement is attributed to Rabbi Joshua ben Levi in Shabbat 89. Rashi also quotes this in his commentary. If Moses was in possession of this secret, why does Rashi have to explain in verse 13, that the meaning of "he stood between the dead and the living and the plague stopped," means that Moses himself had to restrain the angel of death? Rashi there describes an argument in which the angel of death describes himself as G–d's direct messenger, whereas Aaron is described as only Moses' messenger, i.e. an indirect messenger of G–d. Aaron told the angel of death that he was well aware that Moses never does something unless instructed by G–d. He challenged the angel of death to look for confirmation by observing both G–d and Moses at the entrance of the Tabernacle. He invited the angel of death to accompany him to that spot and to ask G–d personally. This is the meaning of the words "Aaron returned to the entrance of the tent to Moses, and the plague had stopped" (17,15). Why would the angel of death kill by means of the plague, when he himself had revealed to Moses how he could be stopped?