Matthew 5:17b (17-26) “I did not come to abolish but to fulfill.”
Murder is that final act of desperation by an angry person. To understand the command, “You shall not commit murder” one must go to what would cause its violation in the first place. Eliminating the cause takes away that motivation to sin (break the command).
For sure anger doesn’t always lead to murder (and there are things we should get angry about) but it always warrants self-examination. Any emotion as a motivation for action should be tempered by reason, even in self-defense.
What Jesus is doing for His disciples (us) is getting them to open their minds to understand the deeper purpose of the command. All the commands have deeper implications that we need to examine as we attempt to obey, so we can fulfill the spirit as well as letter of the guide. In that way we can eventually reach a stage in our lives where there is no need for commands.
This stage is called by various names in scriptures: godliness, Christ-likeness, spiritual and maturity are some. When Peter said, “add to your faith…” it was and is to this end that he spoke, 2 Peter 1:5. See, the Law always had one purpose for those who follow it, Psalm 19:11, but another for those who disobey it, 2 Corinthians 3:6.
Cautiousness involves recognizing the consequences of the decisions I make today.