Ex 11:5 And all the first-born in the land of Egypt shall die, from the first-born of Pharaoh that sitteth upon his throne, even to the first-born of the maid-servant that is behind the mill; and all the first-born of beasts.He caused the incident to happen in the hands of the murderers.
Jn 8:44 Ye are of your father the devil, and the lusts of your father ye will do: he was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it.He ignore the legion of evil angels who are very happy and willing to slaughter them.
Ps 78:49 He cast upon them the fierceness of his anger, wrath, and indignation, and trouble, by sending evil angels among them.All is done for the purpose of saving Israel from slavery in the land of Egypt.
Ps 78:51 And smote all the first-born in Egypt; the chief of their strength in the tabernacles of Ham:
Rom 8:28 And we know that all things work together for good, to them that love God, to them who are the called according to his purpose.So, the next time my Pastor will ask me, who killed the first-born of Egypt? My answer without hesitation would be, the evil angels whose father the devil is a murderer and a liar.