Why did God devoure the sons of Aaron when they took the incense into the alter?

The fire that was to be used in the temple was to be kindled by God, not to use existing fire that was kindled by man.

strange fire: i.e. fire other than that Jehovah had commanded, required, and given from Heaven.

Liviticus:
9:23 And Moses and Aaron went into the tabernacle of the congregation, and came out, and blessed the people: and the glory of the LORD appeared unto all the people.

9:24 And there came a fire out from before the LORD, and consumed upon the altar the burnt offering and the fat: which when all the people saw, they shouted, and fell on their faces.

10:1 And 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.

10:2 And there went out fire from the LORD, and devoured them, and they died before the LORD.