The sin we committed at Peor was bad enough. To this very day we have not purified ourselves; it even brought a plague on the community of the Lord. ¹ 

Then Moses said to Aaron, “Take the censer, put burning coals from the altar in it, place incense on it, and go quickly into the assembly and make atonement for them, for wrath has gone out from the Lord – the plague has begun!”

So Aaron did ¹  as Moses commanded ²  and ran into the middle of the assembly, where the plague was just beginning among the people. So he placed incense on the coals and made atonement for the people.