Their slain shall be cast out. By this circumstance he shews that it will be a great calamity, for if a few persons are "slain," they are committed to the earth; but when so great a multitude is slain at one time, that there are not left as many as are necessary for burying them, there is no thought of interment, and therefore the air is polluted by the stench of their carcases. Hence it is evident, that God is sufficiently powerful to lay low innumerable armies. Perhaps, also, the Prophet intended to heighten the picture of the judgment of God, because to the slaughter of the nations there will be added shame and disgrace, so that they shall be deprived of the honor and duty of burial And the mountains shall melt on account of their blood. Another figure of speech is employed to shew more fully the extent of the slaughter, for the "blood" will flow from "the mountains," as if the very mountains were melted, just as when the waters run down violently after heavy showers, and sweep away the soil along with them. Thus, also, he shows that there will be no means of escape, because the sword will rage as cruelly on the very mountains as on the field of battle.

Their slain also shall be cast out - They would lie unburied. The slaughter Would be so extensive, and the desolation would be so entire, that there would not remain enough to bury the dead (compare the notes at Isaiah 14:19).
And the mountains shall be melted with their blood - The expression here is evidently hyperbolical, and means that as mountains and hills are wasted away by descending showers and impetuous torrents, so the hills would be washed away by the vast quantity of blood that would be shed by the anger of Yahweh.

Their slain also shall be cast out,.... Upon the open fields, and there lie unburied, and become meat for the fowls of heaven, who are invited to them as to a supper, even the supper of the great God, Revelation 19:17,
and their stink shall come up out of their carcasses; so that they shall become loathsome and abominable to the living, and none shall care to come near thereto bury them; an emblem of their loathsome and abominable sins, the cause of this destruction:
and the mountains shall be melted with their blood; an hyperbolical expression, denoting the great number of the slain upon the mountains, and the great quantity of blood shed there; which should run down in large streams, and carry part of them along with it, as large and hasty showers of rain wash away the earth, and carry it along with them; such an hyperbole see in Revelation 14:20.

cast out--unburied (Isaiah 14:19).
melted--washed away as with a descending torrent.

Cast out - Into the fields.