The phrase "all" to is now obsolete, and means "quite," "entirely," as in Chaucer, Spenser, and Milton.

A piece of a millstone - פלח רכב pelach recheb, a piece of a chariot wheel; but the word is used in other places for upper millstones, and is so understood here by the Vulgate, Septuagint, Syriac, and Arabic.
And all to break his skull - A most nonsensical version of ותרץ את גלגלתו vattarits eth gulgolto, which is literally, And she brake, or fractured, his skull. Plutarch, in his life of Pyrrhus, observes that this king was killed at the siege of Thebes, by a piece of a tile, which a woman threw upon his head.

And a certain woman cast a piece of a millstone,.... Of the upper millstone, as the word signifies, which is observed by Jarchi and other Jewish commentators; this with other stones being carried up to the top of the tower, to do what execution they could with them: and a woman observing Abimelech making up to the door of the tower, took up this piece of millstone, and threw it down
upon Abimelech's head, and all to break his skull; she did it with that view, though it may as well be rendered, or "she", or "it broke his skull" (r); it made a fracture in it, which was mortal. Abendana observes, and so others, that that was measure for measure, a righteous retaliation, that as he had slain seventy of his brethren on one stone, he should die by means of a stone.
(r) "et confregit cranium ejus", Junius & Tremellius, Piscator; so Tigurine version.

Mill - stone - Such great stones no doubt they carried up with them, whereby they might defend themselves, or offend those who assaulted them. Here the justice of God is remarkable in suiting the punishment to his sin. He slew his brethren upon a stone, Judges 9:5, and he loseth his own life by a stone.