So, Nadab and Abu offered an unauthorized sacrifice and were immediately killed. It’s pretty intense. The Faithlife Study Bible suggests that the likeliest explanation of their offense is that Aaron was the only one who should have offered the incense.
Maybe they had mixed the incense incorrectly, or maybe they were offering it at the wrong time. In either of these cases, it seems like the offense would be taking the process lightly. Rather than approaching the sacrificial process with the solemnity it deserved. They were, at best, not intentionally giving it their full attention. At worst, they were actively being disrespectful.
If the Study Bible is correct, they were overstepping their position. They were assuming a higher role than they were given, making themselves more important. They were taking a moment that exists for the glory of God and glorifying themselves.
I know this is a very stark report of the incident, but it sounds an awful lot to me like they were trying to claim some attention in the proceedings. It fits with the suggestion that they shouldn’t have made the offering at all. It also fits the punishment. God reacted immediately. The sacrifices were not about the priests preforming them. It was a position of honor, but because it was a position of service. The glory went to God. The attention was on the relationship between the petitioner and God, the offense and atonement or the worship or the gratitude or the promise and the reception. The priests were facilitators, not heroes.
Be careful you don’t wear your Christianity as a badge of honor. Whatever you do, do in love, not for recognition. God might not strike you down, but he knows your heart. Guard it carefully.