“The duty to prosecute, or not to prosecute, is a social and not a legal duty, which depends on the circumstances of each case. It cannot be said that it is a moral duty to prosecute in all cases. The matter depends on considerations, which vary according to each case. But the person who has to act is bound morally to be influenced by no indirect motive. He is morally bound to bring a fair and honest mind to the consideration and to exercise his decision from a sense of duty to himself and others.”