Can you imagine hearing God's voice when walking into the Holy of Hollies. Numbers 7:89 When Moses entered the tent of meeting to speak with the Lord, he heard the voice speaking to him from above the mercy seat that was on the ark of the testimony, from between the two cherubim. He spoke to him that way.
God's instructions of consecrating and anointing Aaron and sons took place and so the ministry of the priests take place.
The disobedience of Aarons sons Nadab and Abihu decided to do their own ritual by taking their own fire pan, put fire in it, placed incense on it and presented unauthorized fire before the Lord which he had not commanded. Then flames leaped from the Lord's presence and burned them to death before the Lord. Leviticus 10:3 This is what the Lord meant when He said; I will show My holiness to those who are near Me, and I will reveal My glory before all the people.
Aaron must have been heart broken for in scripture Lev: 10:3 It says But Aaron remained silent.
Then instructions were given to Aaron by Moses that Eleazar and Ithamar. Do not let your hair hang loose and do not tear you garments or else you would die.
They were also instructed not to go outside the tent of meeting they would die because the Lord's anointing oil is on you. So they obeyed Moses.