Leviticus 16

The Leviticus book deals with the difference between life and death. The tabernacle was where Yahweh had taken up residence, not even Aaron and Moses could enter into it. Nadab and Abihu were struck dead on account of entering the Holy of Holies in an unclean manner. These rules about uncleanness are going to be resolved in Leviticus 16 - which is why those two killed sons of Aaron are mentioned at the beginning of 16.