Why is it those who are quick to disavow God's authority are among the first to ask, "Why does God allow evil?"
If you have any faith, then you will know the answer: for you to choose between it and holiness.
And look to Genesis for God's command:
So it came about in the course of time that Cain brought an offering to the Lord of the fruit of the ground. Abel, on his part also brought of the firstlings of his flock and of their fat portions. And the Lord had regard for Abel and for his offering; but for Cain and for his offering He had no regard. So Cain became very angry and his countenance fell. Then the Lord said to Cain, “Why are you angry? And why has your countenance fallen? If you do well, will not your countenance be lifted up? And if you do not do well, sin is crouching at the door; and its desire is for you, but you must master it.”
You must master it. God created Man in His image. He created us above the animals whose behavior is set by instinct. We are to think. We are to reason. And we are to develop a conscience by which we conduct our lives, one that is hopefully aligned with God's desire to see us become holy.
So do not think we exist at God's whim and have no control. The good news is, we do. The bad news is, we do - and Hell has no one there involuntarily.
Are there demonic influences in the world? Of course. But a society that denies God will deny the Devil, and from that the Prince of Lies gains his greatest strength.
Remember to pray for Adam Lanza, too.