Rashi citing a Midrash ¹ : When Hashem gave the Torah to Yisrael, He distracted the angel of death with other matters, lest he prosecute, and say 'a nation that will deny You 40 days later [via the golden calf], You give to them the Torah?!'

Radak citing his father: This refers to those who died in the Midbar.

Radak citing Ibn Ezra: This refers to the [seven Kena'ani] nations, when Yisrael entered the land ? "Es Eimasi Ashalach Lefanecha v'Hamosi Es Kol ha'Am?" (Shemos 23:27).

Malbim: Dever goes in front of the hidden strength, to plague and kill His enemies.

Rashi: Angels of fire came with Him to Sinai.

Radak: Reshef is like Dever; the matter is repeated in different words. The same applies to "u'Lechumei Reshef" (Devarim 32:24). This is inflammation, which burns and kills quickly. "L'Raglav" is due to Him, like "va'Yvarech Hashem Osecha l'Ragli" (Bereishis 30:30).

Malbim: Reshef goes out due to Him, to destroy them. Even though the Hashgachah is hidden, it is alive and does many acts covertly to make Hashem's enemies submissive via natural evils ? plague and fire.