A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
exo 11:1DEATH OF THE FIRST-BORN THREATENED. (Exo 11:1-10)
the Lord said--rather, "had said unto Moses." It may be inferred, therefore, that he had been apprised that the crisis had now arrived, that the next plague would so effectually humble and alarm the mind of Pharaoh, that he would "thrust them out thence altogether"; and thus the word of Moses (Exo 10:29), must be regarded as a prediction.
exo 11:2Speak now in the ears of the people--These verses, describing the communication which had been made in private to Moses, are inserted here as a parenthesis, and will be considered (Exo 12:35).
exo 11:4Thus saith the Lord, About midnight--Here is recorded the announcement of the last plague made in the most solemn manner to the king, on whose hardened heart all his painful experience had hitherto produced no softening, at least no permanently good effect.
will I go out into the midst of Egypt--language used after the manner of men.
exo 11:5And all the first-born in the land . . . shall die--The time, the suddenness, the dreadful severity of this coming calamity, and the peculiar description of victims, among both men and beasts, on whom it was to fall, would all contribute to aggravate its character.
the maid-servant that is behind the mill--The grinding of the meal for daily use in every household is commonly done by female slaves and is considered the lowest employment. Two portable millstones are used for the purpose, of which the uppermost is turned by a small wooden handle, and during the operation the maid sits behind the mill.
exo 11:6shall be a great cry throughout all the land--In the case of a death, people in the East set up loud wailings, and imagination may conceive what "a great cry" would be raised when death would invade every family in the kingdom.
exo 11:7against any of the children of Israel shall not a dog move his tongue--No town or village in Egypt or in the East generally is free from the nuisance of dogs, who prowl about the streets and make the most hideous noise at any passers-by at night. What an emphatic significance does the knowledge of this circumstance give to this fact in the sacred record, that on the awful night that was coming, when the air should be rent with the piercing shrieks of mourners, so great and universal would be the panic inspired by the hand of God, that not a dog would move his tongue against the children of Israel!
exo 11:8all these thy servants shall . . . bow down themselves unto me--This would be the effect of the universal terror; the hearts of the proudest would be humbled and do reverential homage to God, in the person of His representative.
went out . . . in a great anger--Holy and righteous indignation at the duplicity, repeated falsehood, and hardened impenitence of the king; and this strong emotion was stirred in the bosom of Moses, not at the ill reception given to himself, but the dishonor done to God (Mat 19:8; Eph 4:26).