This report of Nadab and Abihu is one of the most complex stories in the Bible. Why did God kill them? It seems like they had good intentions in serving and praising the Lord, but obviously they failed.
Leviticus 10:9 may offer some explanation as to why these two had to die. Perhaps they had been drinking on the job.
“Self made religion” vs. what God said.
God set unequivocally here at the beginning the standard for punishing inappropriate worship. The major prophets’ reports of how Temple worship ultimately devolved offer stark justifications.