The Law was given to show that we cannot met God’s standard of perfection in our actions – like the one in this passage. Jesus came and showed us that God’s standard of perfection is greater than the law with comments like “if you lust after a women, you have already committed adultery in your heart” (paraphrased).

People learn by analogy – God first gave us the law to show us we cannot live a sinless life. Then He used physical actions as an analogy to address matters of the heart – i.e. lust is a sin just like adultery is sin.

In all cases the goal is to show us that we require intervention from God to get back to God – we cannot do this with our works and righteousness. The action in your passage is the act of following the explicit rules for worship – the analogy in my opinion is to Acts chapter 5 with Ananias and Sapphira. One is a matter of action, the other a matter of the heart. In both cases, man fails and requires a divine solution.

Balanced with Romans 6:1,2 – shall we continue to sin for the sake of Grace – certainly not. (paraphrased), We cannot and should not attempt to follow rules in order to please God – we will fail.

The Bible didn’t say because of sin but the nature of the sacrifice.

“Now Nadab and Abihu, the sons of Aaron, each took his censer and put fire in it and laid incense on it and offered a unauthorized fire before the Lord, which he had not commanded them. 2 And fire came out from before the Lord and consumed them, and they died before the Lord.”