The Yalkut (524 towards the end) quotes a view that Nadav and Avihu were punished because they did not marry. Rav Levi said: Many girls remained agunos because they sat waiting to marry them! And what did Nadav and Avihu say [i.e. what was their excuse]? They said, "Our father's brother is king; our mother's brother is a prince; our father is kohen gadol; we are assistant high priests -- what girl is good enough for us?!"
There is an obvious musar haskel here for those in the shidduch world, but you don't need me to spell that out.
A less obvious lesson: R' Henoch Leibowitz z"l in his Ch. Lev asks: why were Nadav and Avihu punished because these girls chose to remain agunos, hoping they would marry them? Let them go marry someone else!
R' Henoch says an unbelievable answer. Even though Nadav and Avihu surely did not intend for these girls to remain unmarried, and even though the girls had a hand in deciding their own fate, since their plight was a result of Nadav and Avihu's actions, Nadav and Avihu share in the burden of culpability.
I was taken aback by this. You mean if someone else makes a bad decision, so long as I had a hand in putting them in the situation that caused that decision to be made -- I was lifnei iveir and enabled them to exercise their own stupidity -- I am culpable? Apparently there is a price to be paid even for the most unintended of consequences. Pretty harsh din v'cheshbon even for a musarist to suggest. I've been trying to think of a way to read this that would temper the message a bit, but haven't come up with a good approach yet.