B’nai Yisrael were wary of the Ktoret (incense) for a few reasons:
Aharon’s two sons, Nadav and Avihu died because of the incense as we see in Parshat Shmini, Vayikra 10:1-2: “The sons of Aharon, Nadav and Avihu, each took his fire pan, they put fire in them and placed incense upon it; and they brought before God an alien fire that He had not commanded them. A fire came forth from before God and consumed them, and they died before God.”
What did they do that was so bad?
According to Sifra, they erred by bringing their own incense into the Kodesh HaKodashim (Holy of Holies) which was off limits to everyone except for the Kohen Gadol (High Priest) on Yom Kippur.
Rabbeinu Bachya says that they didn’t go into the Kodesh HaKodashim, rather they offered the daily incense on the inner altar without being commanded.
According to Rabbi Eliezer, the problem was that they were guilty of making a Halachic decision without consulting their teacher, Moshe.
In this case, the incense itself wasn’t the problem. The issue was that Nadav and Avihu took it upon themselves to decide when and where to bring it.
In Parshat Korach (Bamidbar 16:6-7), Moshe offered Korach and his followers a challenge: “Do this: Take for yourselves fire-pans- Korach and his assembly- and put fire in them and place incense upon them before God tomorrow. Then the man that God will choose- he is the holy one…”
Korach and his followers would prove themselves if their incense was accepted, but if not they would die. Moshe was hoping that Korach and his men would withdraw from their rebellion rather than take a chance. Unfortunately, they went through with it and we see in Bamidbar 16:35 “A flame came forth from God and consumed the 250 men who were offering the incense.”
We see from these two cases why B’nai Yisrael thought that incense was deadly. However, Rashi points out (17:13) that incense in itself does not kill, it is sinfulness that kills. To show them that incense can also have a positive effect and save lives, Moshe instructed Aharon during the plague (Bamidbar 17:11-12):
“Take the fire pan and put fire from atop the altar on it and place incense on it, and go quickly to the congregation and atone for them for anger has come forth from before God; the plague has commenced.” Aharon took as Moshe had spoken and ran to the midst of the congregation and behold! The plague had begun among the people. He placed the incense and provided atonement for the people.
Here the incense was used for a positive reason. In verse 13 the plague stopped. In verse 14 the final death toll of the plague was totaled at 14,700 (not counting the 250 people who died due to Korach’s rebellion) and in verse 15 Aharon returned to Moshe, to the entrance of Ohel Moed and the plague stopped.
Why does the Torah list twice (verses 13 and 15) that the plague stopped?
According to Sforno, the first time that it says that the plague stopped (verse 13) refers to the fact that nobody else got sick. The second time that it says that the plague stopped (verse 15) indicates that all those who were ill were cured.
May all who are infected be healed and may we see no new cases of the plague of Covid-19.