בקרבתם לפני ה’ וימותו (אחרי טז:א)

A Yid who wants to become spiritually closer to Hashem can not do it alone. He needs the help of a tzaddik from his generation to guide him in the right direction.  Otherwise, one can transgress and fall from the right path.  Nadav and Avihi wanted to become closer to Hashem through their own merits and strengths, without the help of Moshe and Aaron, בקרבתם לפני ה’ – since their approach was on their own, וימותו  – therefore, the punishment of death came upon them.