When God specifies something, He expects and demands exactly what He said.
We previously covered GOD's miraculous consumption of the sacrifice offered by Aaron and his sons as their first priestly actions on behalf of the people of Israel.
Unfortunately, tragedy immediately followed this miracle.
9:22 Then Aaron lifted his hand toward the people, blessed them, and came down from offering the sin offering, the whole burnt offerings, and peace offerings. 23 After this, Moses and Aaron went into the tabernacle of testimony, and came out, and blessed all the people. Then the glory of the Lord appeared to all the people; 24 and fire came out from the Lord, and consumed the things on the altar, both the whole burnt offerings and the fat. When all the people saw this, they were amazed and fell on their faces.
10:1 Now Nadab and Abihu, Aaron's sons, each took his censer and put fire in it, put incense on it, and offered strange fire before the Lord, which He had not commanded them.
2 So fire went out from the Lord and devoured them, and they died before the Lord. 3 Then Moses said to Aaron, “This is what the Lord spoke, saying, ‘By those who come near Me I shall be sanctified; and before all the congregation I shall be glorified.’” So Aaron was greatly distressed. 4 Moses then called Mishael and Elzaphan, the sons of Uzziel, Aaron's uncle, and said to them, “Come near, carry your brethren from the presence of the sanctuary outside the camp.” 5 So they came near and carried them by their tunics outside the camp, as Moses said. 6 Then Moses said to Aaron and to Eleazar and Ithamar, his sons, “Do not uncover your heads, nor take off your garments, lest you die, and wrath come upon all the congregation. But let your brethren, the whole house of Israel, bewail the burning the Lord kindled. 7 You shall not go out from the door of the tabernacle of testimony, lest you die, for the anointing oil of the Lord is upon you.” So they did according to Moses’ word.
(Leviticus 9:22-10:7 OSB)
Aaron and his four sons, including Nadab and Abihu, had just completed a week-long period wherein they strictly followed precise instructions GOD had given them as relayed by Moses.
Moses had told the new priests exactly what GOD wanted done and exactly how He wanted them done.
In these activities, GOD was specific.
With many things in life, GOD grants us freedom to make our own decisions as we see fit, provided we behave in faith and making an effort to please Him.
But when GOD specifies something, it is a matter of rebellion for us to fail to do exactly as GOD specified.
GOD knew Nadab and Abihu's heart. He knew that they were not properly honoring Him with their actions. And He burned them up by fire because of it.
GOD had to impress upon Israel that, when they approached Him, they had to do exactly as He specified, that inventing their own practices outside of what GOD had prescribed would not be acceptable to Him.