The "Hitler" analogy is not merely hypothetical, it is totally irrelevant in relation to John Newton. A more relevant analogy might be with the man I mentioned in an earlier post - Oscar Schindler. A man who was far from being flawless, living in a desperate time, a mixture of altruism and self-seeking, and a saviour for a lot of people. I don't think many people would have problems of principle in singing a song he might have written,

Hitler was painter, but a pretty poor one. If he had been a great artist would we feel we had a duty to destroy his painting? An interesting question to toy with, but not one we need to try to settle.

After all if Hitler had been a great artist he would not have been Hitler, and who know how his life would have unfolded. At a much lower level of evildoing it is notable that the fact that the painter Caravaggio appears to have been a murderer has not led to his paintings being shunned.