Matthew 5:21
"You have heard that it was said to those of old, 'You shall not murder; and whoever murders will be liable to judgment.'
it was said to those of old
Jesus begins here to use this formulation to identify when people were teaching from the Old Testament but were not going far enough in their application.
The commandment is first given at Exodus 20:23, and the rules and punishment for murder are addressed at many points in the Law. A quick review of the passages shows they are all reactions to the fact a murder has already been committed.
Until Jesus said this, there was little teaching being done in the way of prevention and forethought. Many even taught that the only sin was murder itself; as long as you don't kill him, they reasoned, you're OK!