The ten commandments in Exodus 20 have no content. We are not told what idolatry, honouring parents, murder, adultery, stealing, etc. are. But every nation and culture interprets these categories of moral judgment in different ways (see the first chapter of the book on Adultery on the web site). And the way the ten categories of moral judgment are interpreted tells us what the culture of that nation was at that time. When Jesus came he said "But I say unto you" concerning the way the categories were interpreted (see Matthew 5:21-48) in ancient times.
We can deduce that he says "But I say unto you" concerning the culture and moral judgments of every nation. So we can look at our own messy national history and culture, and imagine what Jesus' "But I say unto you" would be.