Why does God not kill the devil?

God is not a murderor. He wishes for only peace and he examplifies this throughout his mercy.

Answer

God does not kill the Devil because it would negate God's intentions in creating us.

When God gave man free will, it allowed evil to exist and created a divide between the perfect God and imperfect creation.The role of the Satan is to enforce that divide.

Contrary to popular belief (mostly as the result of Paul's misunderstanding of both Judaism and Christianity when he Hellenized it and encorporated many pagan ideas later in the New Testament), the Satan is an agent OF God not acting in opposition. While it's true that "Satan" (properly Shaitan) translates as "the adversary" it is not of God, but of man. Think of Satan as being like a white blood cell, purging creation of illness and disease.

By over coming temptations and enduring trials and tribulation, mankind proves it's holiness and is admitted back into the divine presence so thatnwe can become as God intended- truly in his image. If we were to be admitted into the presence without first being purified, we would be totally annihilated. Even Jesus had to endure trials and temptations to prove his holiness before he could be admitted back into the presence of his father.

Man creates evil, not Satan. Satan tempts us but can not lift so much as a finger against us, so why destroy Satan? It is our free will and our ability to make bad choices that cause the evil that we must all endure.