Moses gave a law that most of us learn as a kid. Jesus comes and says it’s not just about the action, instead it is mainly about the heart. Don’t murder, of course. But don’t even be angry, rather be reconciled. Jesus could say this because He is the greatest reconciler of all time.
The greatest sermon ever recorded was the sermon Jesus gave on a mountainside. The Sermon on the Mount starts and ends with the Gospel. Everything in-between is what it looks like to live a Gospel-infected life. Jesus gives us practical life lessons, not to prove we are Christ followers but because we are Christ followers.