A fair question, Steve. Too often we hear the tale that Newton's "dramatic conversion" led to his immediately giving up the slave trade, which as you say is not true at all.

My own feeling is that anyone can sing any song, regardless of what kind of person wrote it. For many songs, we'll never know if the person who wrote it was good or evil. Also, meaning is not encoded into a song once, to remain forever static thereafter. To the extent that the text and context allows it, we get to decide what songs mean. "Amazing Grace" can be an anti-slavery song, or it can have nothing to do with slavery, because its text is really about conversion, but does not describe the specifics.