ותצא אש מלפני יקוק ותאכל אותם וימתו לפני יקוק
A fire went forth from before Hashem, and consumed [Nadav and Avihu], and they died before Hashem
At the conclusion of the Mishkan’s inauguration, the people were beset with tragedy. Nadav and Avihu, two of Aharon’s children, who were leaders of the Jewish people, died. They had volunteered an unrequested fire offering to Hashem, and perished instantly. The consequences of their actions seem too severe for their “crime”. Indeed, what they did seems meritorious. They were displaying their devotion to Hashem, and their joy at the opportunity to express it. As a result, many explanations are given for what their real crime was.