Eish Min Hashamaim

The Gemara in Yoma 53A states that Bnei Aharon were killed because they were Moreh Halacha Bifnei Raban. That they did not wait for Moshe rabeinu to Pasken but they did so themselves. The Gemara says that the halacha they taught was that "Even though a fire comes down from heaven, it is still a Mitzvah to bring fire for the Ma'aracha from the (Hedyot)corporeal".

I would like to suggest that there was a parallel here and a Mida Kineged Mida. The Bnei Aharon felt that although you brought fire from the Hedyot the real work was being done by the fire from above, this led them to also believe that although we are commanded to learn and be mechadesh torah on our own the real torah is from the Rebbe given down from moshe rabenu. Therefore they felt that being moreh hora'ah befnei raban is not such a sin because it is not having any real effect however they were shown the truth by a fire coming down from heaven and killing them that although the real work is done from heaven there is still a responsibility on earth and that is a real responsibility and because they didn't recognize that they were killed by a fire from heaven