The Sfas Emes says in the name of his grandfather, the Chiddushei Harim, regarding the children of Aharon HaKohen who lost their lives as they brought ketores in the Mishkan: The pasuk says, ויקריבו לפני ה' אש זרה אשר לא צוה אות, and they brought before Hashem an alien fire that He had not commanded them (Vayikra 10:1). Explained the Chiddushei Harim, in every mitzvah that we’re commanded to do lies a chiyus, an animating force, from the commandment of Hashem. When we carry out the command of the Ribbono shel Olam, we become His emissaries to fulfill His wishes. This gives a Yid the chiyus to carry out the mitzvah.
The problem of Nadav and Avihu, explains the Gerrer Rebbe, was that they lacked the command of Hashem. It was אשר לא צוה אותם, and this is why they were consumed and were taken from This World. The actual act that they did was great and holy—they brought ketores, which is a tremendous avodah—but they were unable to connect to the essence of the mitzvah, to the ratzon Hashem, because there was no command for them to bring it.
