פירוש על במדבר 17:11

וכפר עליהם AND MAKE AN EXPIATION FOR THEM — This secret the Angel of Death communicated to him (Moses) when he went up unto heaven to receive the Torah, viz., that the incense holds back the plague — as is related in Treatise Shabbat 89a.

קח את המחתה, "take the censer, etc." Inasmuch as the people had claimed that the reason the 250 men had died was due to Moses having challenged them to offer the deadly incense, Moses now wanted to demonstrate that incense is far from deadly; on the contrary, this very "deadly" incense would now save the people's lives. The reason the incense had appeared to be "deadly" was because the wrong people had offered it at the wrong time and at the wrong place. Our sages in Shabbat 89 claim that Moses had learned the life-saving property of the incense offering from the angel of death himself when he had been on Mount Sinai waiting to receive the Tablets and the Torah.

ושים קטורת, to teach the people the lesson that the same incense which is causing death when used by non priests is a remedy when used by the priests. This is further priests that only the priests are worthy of performing sacrificial service.