Mr. Newton is long dead. He tried to come to terms with the errors in his life that he saw, which is the best that any of us can do. None of us are in a position to judge him, in part at least, because none of us know what was in his heart.

Slavery still exists in this world. Both children and adults are still sold into bondage in West Africa, there are slaves held in Sudan, Pakistan, Haiti, India, and many other places. Check this for an overview-Slavery in the Modern World

Whether you sing "Amazing Grace" or not has no effect on the situation of human beings who are being held in slavery. There are a lot of other actions that you can take which may make a difference. The choice, as always, is yours alone.