So what did Nadab and Abihu do that angered God so much he burned them to death in the tabernacle? Leviticus 10 says only that they offered "strange fire." But what does that mean?
Obviously, they didn't follow directions for proper sacrifice, which had just been given in exacting detail. But were they being rebellious, or did they just think it didn't matter that much? The text doesn't say -- which makes for interesting conjecture.
But the real point is that they didn't do exactly as told. Exactly what they did do isn't as important as the fact that they thought they could monkey with God's commands and he wouldn't really care.
Many of the same questions people have about Nadab and Abihu's actions are brought out in Genesis 4, where Cain and Abel offer sacrifices. God regards Abel's sacrifice favorably, while he had no regard for Cain's.
The text hints that Cain didn't offer a proper sacrifice, but it doesn't say what was wrong with his -- or what was right with Abel's. Some have argued that because Abel's was meat, it was a foreshadowing of Christ's sacrifice. Other's have pointed out that the text notes Abel gave of the firstborn, while Cain's is not referred to as firstfruits.
Despite these theories, the interesting thing, again, is the vagueness. Readers of the text are left to wonder exactly what Cain, Nadab and Abihu did wrong, and as a result they must be even more vigilant not to repeat their mistakes.
It's like if you started a new job and your boss gave you a list of do's and don'ts, warning that he tolerates no rule-breaking. Then you find out a guy was fired one week after getting the rules, but you aren't told what he did.
What do you do? You read the rules carefully and try to follow all of them to the letter.
But if someone tells you the guy was fired for embezzlement you might get a little more slack. Of course the boss would fire him for embezzlement, but he would never fire you for being habitually 10 minutes late, would he?
So we are in our spiritual lives. Sure, God struck Ananias and Sapphira dead for lying to the Holy Spirit, but my copying of software isn't like that. After all, Bill Gates has all the money he needs, right? And my cussing those idiots on the freeway isn't all that bad since it is their stupidity that's fueling road rage. If not for them I'd be a peach.