Leviticus 10:1–2 shares this sobering account, stating, “Aaron's sons Nadab and Abihu took their censers, put fire in them and added incense; and they offered unauthorized fire before the LORD, contrary to his command. So fire came out from the presence of the LORD and consumed them, and they died before the LORD.”
You mentioned Aaron's son before. One thing specifically to note about the OT is the way God interacts with the people of Israel.
In the OT, God's interaction with the Israelites , or one of the ways He interacted, was through the Ark and the Temple. We don't have a modern day equivalent to that (praying and being in the presence of God isn't quite the same deal as the Ark).
When people in the OT, whether it be Aaron's son, Uzziah, or the Caananites, disrespect the Altar of God, they not only interact with the presence of God on earth, they do so in a way that spits in His face in a way that doesn't really correlate to a 21st century mindset. They're not just dishonoring him, they were doing so in a way they ought to know not to.
It would be like an astronaut studying the sun from a ship, flying too close because he's scornful of the idea that it might hurt him.
R/atheism or any amount of pagans nowadays who might mock or disrespect God or the idea of God, have absolutely nothing on this.
It's simply a matter of interacting wrongly with a manifestation of the presence of God that had fatal consequences. We can't interact with the pure Presence as mortal humans; because of Jesus we're saved but until we're transfigured at the end we still would need His direct protection not to burn up, if we got to stand in a direct manifestation of the same magnitude as the Ark.