Nadab and Abihu were the sons of Aaron who were famously incinerated because they tried to approach the Lord with strange incense (Lev. 10:1-2). It seems to me that they were trying to accomplish something, or see something. I don’t think they were judged just because they got the formula wrong when they were mixing the incense up. One possibility is that they wanted an incense that would enable them to see something that would satisfy their curiosity. There appears to have been a back story involving alcohol because immediately after this, the Lord prohibits priests from drinking while on duty (Lev. 10:9). Whenever a disaster happens, and a new rule appears right afterwards, it is reasonable to assume that the rule has something to do with the disaster.
But it is striking that there was also a prequel (Ex. 24:1). Nadab and Abihu were specifically invited by name up on the mountain by the Lord to sit down and eat in the presence of the Lord.
“And he said unto Moses, Come up unto the Lord, thou, and Aaron, Nadab, and Abihu, and seventy of the elders of Israel; and worship ye afar off . . . Then went up Moses, and Aaron, Nadab, and Abihu, and seventy of the elders of Israel: And they saw the God of Israel: and there was under his feet as it were a paved work of a sapphire stone, and as it were the body of heaven in his clearness. And upon the nobles of the children of Israel he laid not his hand: also they saw God, and did eat and drink.” (Ex. 24:1,9-11).
If this reconstruction is correct, this means that Nadab and Abihu tried to seize what they had previously been granted by grace. Nadab and Abihu were given what very few people have been given in the history of the entire world. But what they had previously had by gift, they tried to grab by manipulation later, and came to a sorry end.
One lesson is that starting well is not the same thing as finishing well. Another related point is that yesterday’s gift is not today’s gift. Today’s gift must be received as a gift today.