Sin isn't necessary, but it's a side effect. God existed for eternity past without sin and yet he could've chosen to sin. And the "evil" talked about in Isaiah isn't moral evil, it's natural evil, as in natural disasters. That doesn't mean that every natural disaster is puppeteered by God, but he does use them sometimes for his purposes. Mostly judgement. When you are both again, the Holy Spirit dwells inside you. He works on you through the conviction of sin to help rid you of sinful desire. So when you're in heaven, the combination of the holy spirit and being in the literal presence of God the Father and our saviour Jesus Christ, we will have no desire to sin, but we will still have a choice.