A common question, but the answers I've heard are mostly not to the point, useless. Such as at the last Friday's CCCNY Bible study. The pastor's reply was: We are not qualified to judge, it is God's decision, leave it to God.
Yes, we are not better or equal to God. But that's beside the point. The question, especially when asked by a non-Christian, rises from motive questioning God's fairness. So, the question was not answered.
This is a key to identify who has the burden to preach the Gospel. One who has such burden would not play the "Leave it to God" card so hastily. Because the key to answering the question lies in the understanding of sin. One must understand, even for the one who is answering, how serious sin has damaged the relationship between the sinner and God. How we as sinners, are enemies of God, all of us. Once that is understood, it is not likely that one would consider someone worse than a murder to be pardoned so easily. Then, we see the grace of God in full understanding.