He had not commanded them

“And Nadab and Abihu, the sons of Aaron, each took his censer and put fire in it, and put incense on it, and offered strange and unholy fire before the Lord, as He had not commanded them. And there came forth fire from before the Lord and killed them, and they died before the Lord” Leviticus 10:1-2 (AMPC)

Nadab and Abihu sons of Aaron were assigned a role to help their father the then high priest. They were to light their censers from the fire which their father ignite from the altar of God, in the presence of God. 

 However, the duo had the” better idea,” and they took their light from the fire that Aaron used to boil the meat for the sacrifice. 

Maybe they thought fire is fire anyway!

That made the fire in the censer unclean and it displeased God. 

The judgment of God on their disobedience was obvious and instant.

Do what God has commanded you to do.

Don’t just do what others are doing.

Possibly they were not patient enough for their father to finish the process of starting the fire from the altar and they wanted to show off to the crowd that they got the fire too. 

Pride usually precedes destruction.

Don’t allow peer pressure to make you do favorite things that God has not commanded. 

Saul lost his kingdom because he offered a sacrifice that was meant for Samuel only to perform because he wanted to impress the people of Israel at that time 

“ Samuel said, What have you done? Saul said Because I saw that the people were scattering from me,… So I forced myself to offer a burnt offering. And Samuel said to Saul, You have done foolishly! You have not kept the commandment of the Lord your God which He commanded you; for the Lord would have established your kingdom over Israel forever;” Samuel 13:11-13 (AMPC)

Our Lord Jesus Christ only carried out the things God the Father commanded Him to do 

“ So Jesus said, “I speak to you timeless truth. The Son is not able to do anything from himself or through my own initiative. I only do the works that I see the Father doing, for the Son does the same works as his Father.” John 5:19 (TPT)

Do what God has commanded.