The relationship between the letter and the spirit of the law is an important concept to understand. The letter of the law refers to the commands and regulations, while the spirit of the law refers to the underlying principles and intent behind those commands. they both are intertwined and mutually reinforcing and are inseperable.

When Jesus spoke about the letter and the spirit of the law, He was addressing the distorted interpretation and application of the law by the religious leaders of His time. They focused on obedience with the letter of the law while neglecting the inner attitudes and motivations that should accompany it. Jesus emphasized the importance of not only obeying the outward commands but also embracing the principles of love, justice, mercy, and righteousness that underlie those commands.

In regards to your example of killing, stealing, and committing adultery, it is clear that the letter and spirit of the law both prohibit such actions. The commandments explicitly forbid these behaviours, and the spirit of the law calls for love, respect, and consideration for others. We cannot fulfill the spirit of the law without also adhering to its letter.

Sabbath-keeping can be a means of embracing the spirit of the law. Keeping the Sabbath is not just about observing a day of rest; it is about recognizing and honouring God as the Creator and Redeemer, and it is about expressing love for God and others. It involves both the outward observance and the internal conviction of reverence and worship.

In summary, the letter and the spirit of the law are inseparable. Keeping the spirit of the law requires adherence to its letter, and fulfilling the letter of the law should be done with the spirit of love, righteousness, and sincere devotion to God and others.