Exod 12:2 ‘For I will pass through the land of Egypt on that night, and will strike all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the Lord."

According to you the above is solely a matter of God allowing Satan to kill all the firstborn in Egypt. Have you considered what would happen if Satan decided that he was simply not going to do the Lord's bidding ? That since God had already said it therefore Satan refuses to do it. Or alternatively he will kill a few of them but not bother to kill so many that it will look like what God said has come to pass ?