Here's what good ole' J. Vernon McGee says on the last verse:
Zoroastrianism began in Persia. It teaches that Mazda is the god of light. God says He creates light, and that it is no god. The Persians were getting very close to the truth. Many have wondered why they worshiped one god in the midst of idolatry. Well, you must remember that they came in contact with the nation Israel, and Israel was a witness to the world. In Zoroastrianism darkness was Ahriman, the god of evil. God takes responsibility for creating the darkness also.
"And create evil" -- the word evil does not mean wickedness in this instance, but rather "sorrow, difficulties, or tragedies" -- those things which are the fruit of evil, the fruit of sin. This is the Old Testament way of saying, "The wages of sin is death . . ." (Rom. 6:23). If you indulge in sin, there will be a payday for it!
By the way, let me introduce something else at this point, since we are living in a day when it is said that good and evil are relative terms, that whatever you think is good, is good. The argument is put forth: The Bible says "Thou shalt not kill" and "Thou shalt not steal" (Exod. 20:13, 15). But what is the Bible? Who should obey it? Or why should we listen to the God of the Bible?
The Lord has another very cogent argument. God says that if you indulge in sin, you will find that sin has its payday. It pays a full wage, by the way. This is what God is saying through Isaiah. God has so created the universe that when you break over the bounds that He has set, you don't need a judge, a hangman's noose, or an electric chair; God will take care of it.
He says, therefore, that He is the One who creates light and darkness. He is answering Zoroastrianism which worshiped the god of light. God says, "I want you to know that light is no god; I created it."
—J. Vernon McGee's Thru The Bible