27:1 That day is a reference to the end of the evil world as we know it. In ancient Aramean (Ugaritic) literature, leviathan was a seven-headed monster, the enemy of God’s created order. Thus, Isaiah is comparing God’s slaughter of the wicked to the conquering of a great enemy. Although evil is a powerful foe, God will crush it and abolish it from the earth forever.
27:9 Only God can take away sin, but to be driven out of the land was considered the penalty that would purify God’s people. Deuteronomy 28:49-52, 64 explains God’s warning about these consequences.
27:11 Isaiah compares the state of Israel’s spiritual life with dead branches of a tree that are broken off and used to make fires. Trees in Scripture often represent spiritual life. The trunk is the channel of strength from God; the branches are the people who serve him. Tree branches sometimes waver and blow in the wind. Like Israel, they may dry up from internal rottenness and become useless for anything except building a fire. What kind of branch are you? If you are withering spiritually, check to see if you are firmly attached to God.