"and the L–rd smote every first-born": I might think through an angel or through an emissary. It is, therefore, written (Ibid. 12:12) "and I shall smite every first-born, etc." — not through an angel and not through an emissary.
Perhaps an Angel Carried Out the Slaughter, or Some Other Celestial
Mekhilta Tractate Pischa 13:4