Gemara

Beraisa: "V'Nosnu Benei Aharon ha'Kohen Esh Al ha'Mizbe'ach" - even though fire descends from Shamayim, it is a Mitzvah for people to bring fire.

53a - Beraisa - R. Eliezer: Aharon's sons were killed because they (implicitly) ruled in front of Moshe their Rebbi. They expounded "V'Nosnu..." - even though fire descends from Shamayim, it is a Mitzvah for people to bring it.

Question: Ketores is burned on the inner Mizbe'ach. Heavenly fire was only on the outer Mizbe'ach!

Answer (Tosfos Yeshanim): They took fire to burn Nachshon's Ketores on the inner Mizbe'ach, which is where the Ketores of the Nesi'im was burned. This also explains why two incense pans were needed.

Ra'avad (Toras Kohanim Shemini 1:22): They did not burn the daily Ketores; surely that was burned in its proper time, before the limbs of the Tamid! Rather, they took additional Ketores to burn on the fire they brought.

Toras Kohanim (ibid. 21): When fire descended from Shamayim, people joyously sang Shirah. Nadav and Avihu were punished. Some say that they were punished for transgressing at Sinai, when they anticipated being the leaders after Moshe and Aharon would die.

Ra'avad: All agree that they died for their sin now. The latter opinion holds that Hash-m gave them the opportunity to sin now because of their earlier sin.

Toras Kohanim (ibid. 22): Alternatively, when Nadav and Avihu saw that all the Korbanos were offered but fire did not descend from Shamayim, they decided that it was necessary to bring fire to burn the Korbanos. They brought fire into the Kodesh ha'Kodoshim; Hash-m brought fire to burn them.

Ra'avad: This opinion holds that they were killed at the same time that the Shechinah and fire for the Mizbe'ach came down. The first opinion holds that they brought fire to burn the Ketores on the inner Mizbe'ach.

Toras Kohanim (ibid. 32): Suggestion: "Va'Yikrevu...Esh Zarah..." - perhaps it was truly Zarah (improper)!

Rejection (R. Yishmael): "Asher Lo Tzivah Osam" - their sin was to act without Etzah (taking counsel; Ra'avad's text - b'Lo Itah, in the wrong time).

Ra'avad: We must say that this was after Heavenly fire descended - otherwise it was indeed Esh Zarah! They offered Ketores, unaware that Aharon already offered it.

R. Akiva says, it was indeed Esh Zarah from an oven!

Question: If so, "Asher Lo Tzivah Osam" is superfluous!

Answer: It teaches that they were punished also for not asking Moshe.

Ra'avad: This was after Heavenly fire descended; they should have taken fire from the Mizbe'ach.

R. Eliezer: They were punished only for ruling in front of Moshe their Rebbi.

Ra'avad: They knew that even though fire descends from Shamayim, it is a Mitzvah for people to bring fire. They could not fulfill this on the outer Mizbe'ach, for the Heavenly fire was consuming the Korbanos, so they brought fire inside with Ketores.

Rishonim

Rambam (Hilchos Temidim u'Musafim 2:1): Even though fire descends from Shamayim, it is a Mitzvah for people to bring it - "V'Nosnu..."

Sefer ha'Chinuch (Mitzvah 132): Hash-m usually covers His miracles and makes them close to natural. Clever people will realize that this testifies to His greatness and our lowliness. It appears that normally people did not see the fire descending from Shamayim, except on the eighth day of the Milu'im, and regarding Gid'on and Mano'ach (and Eliyahu).