Being before instructed of her mother - Not before she danced, but afterward, and before she made the request of Herod.
See Mark 6:24. The only appearance of what was right in the whole transaction was her honoring her mother by consulting her, but in this she only intended to accomplish the purposes of wickedness more effectively.
In a charger - The original word means a large platter on which food is placed. We should have supposed that she would have been struck with abhorrence at such a direction from her mother; but she seems to have been gratified. John, by his faithfulness, had offended the whole family, and here was ample opportunity for an adulterous mother and her dissolute child to gratify their resentment. It was customary for princes to require the heads of persons ordered for execution to be brought to them. For this there were two reasons:
1. To gratify their resentment - to feast their eyes on the proof that their enemy was dead; and,
2. To ascertain the fact that the sentence had been executed.
There is a similar instance in Roman history of a woman requiring the head of an enemy to be brought to her. Agrippina, the mother of Nero, who was afterward emperor, sent an officer to put to death Lollia Paulina, who had been her rival for the imperial dignity. When Lollia's head was brought to her, not knowing it at first, she examined it with her own hands until she perceived some particular feature by which the lady was distinguished.

Give me here John Baptist's head in a charger - The word charger formerly signified a large dish, bowl, or drinking cup: the Saxon has a dish, Tindal, a platter; any thing is better than charger, which never conveyed much meaning, and now conveys none. The evangelist says she was instructed before, by her mother, to ask the Baptist's head! What a most infernal mother, to give such instructions to her child! and what a promising daughter to receive them! What a present for a young lady! - the bloody head of the murdered forerunner of Jesus! and what a gratification for an adulterous wife, and incestuous mother! The disturber of her illicit pleasures, and the troubler of her brother-husband's conscience, is no more! Short, however, was their glorying! See on Matthew 14:3 (note).

And she being before instructed of her mother,.... What request to make; for as Mark says, "she went forth" to her mother immediately, as soon as she had received the king's promise, and took advice of her, what she should ask; who bid her ask for the head of John the Baptist; and accordingly she went in, "straightway with haste unto the king", as the same evangelist observes, to take him at his word, and whilst he was in the mood; being urged and hastened on by her mother, who was eager to satisfy her revenge on John; and said,
give me here John Baptist's head in a charger: she desires his head, and this to be brought to her in a large dish, that her mother might be sure of his death; and have an opportunity of insulting that mouth and tongue, that had spoke against her incestuous marriage: and she desires to have it given "here", in that very place, at that very time, where, and while the company was together, who were witnesses of the king's promise and oath; and this she did, lest when the festival was over, and he was out of his cups, he should repent of his folly and rashness. The mother and daughter seem to be much alike, both for lasciviousness, revenge, and cruelty: and if what the historian says (w) be true, that this same person Salome, the daughter of Herodias, as she walked over a river which was frozen in the winter season, the ice broke, and she fell in, and the pieces of ice cut off her head; the "lex talionis", the law of retaliation, was righteously executed on her.
(w) Nicephorus, Hist. l. 1. c. 20.

Give me John the Baptist's head in a charger. Mark tells us that she went to consult her mother before she made her request. That vile woman was prepared with an answer. Indeed, she had manipulated the whole affair so as to secure Herod's consent to the murder of John.
In a charger. An old English word for a large dish, so called from the load it sustained.

Being before instructed by her mother - Both as to the matter and manner of her petition: She said, Give me here - Fearing if he had time to consider, he would not do it: John the Baptist's head in a charger - A large dish or bowl.