And Aharon took as Moshe had spoken, and he ran into the midst of the assembly, and behold the plague had begun among the people; and he put on the incense and atoned for the people.
Rashi / Shabbos 89a: the secret that incense stops the plague was given over by the angel of death when Moshe ascended.
English: And he ran into the midst of the assembly — and he put on the incense and atoned for the people.
Ketores as atonement that stops the plague.
The same incense that killed the unauthorized becomes, in the hand of Aharon, the means that stops death.