God burned Aaron's sons to death for offering "strange fire before the Lord"

The story begins with Aaron's sons, Nadab and Abihu, burning "strange fire before the Lord," some kind of weird incense that God didn't like very much.

Nadab and Abihu, the sons of Aaron, took either of them his censer, and put fire therein, and put incense thereon, and offered strange fire before the LORD, which he commanded them not. Leviticus 10.1

So God burned them to death.

And there went out fire from the LORD, and devoured them, and they died before the LORD. 10.2

And here's what Moses said to Aaron afterwards.

Then Moses said unto Aaron, This is it that the LORD spake, saying, I will be sanctified in them that come nigh me, and before all the people I will be glorified. 10.3a

God burned Aaron's sons alive so that God would "be sanctified in them" and "glorified."

That helps.

Moses warned Aaron not to mourn the death of his sons (by uncovering his head or tearing his clothes) or God would kill him too, along with all the people.

Moses said unto Aaron ... Uncover not your heads, neither rend your clothes; lest ye die, and lest wrath come upon all the people. 10.6

So Aaron did as he was told, and watched in silence as his sons were burned to death by God.

And Aaron held his peace. 10.3b

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The Friendly Atheist - How many people did God kill in the Bible?

Aaron's sons