David justifies his decision to spare Saul's life by drawing on deep-rooted principles of divine justice and human nature. He grounds his reasoning in an ancient proverb, a concept that carries several layers of meaning. The primary approach among commentators is that David is quoting a well-worn saying passed down through generations [מצודת ציון, רד״ק, ביאור שטיינזלץ]. Some trace this saying back to a specific historical event, viewing it as the very first proverb in human history. It originated with the story of Cain and Abel, noting how the wicked Cain was ultimately killed by Lamech, another wicked man from his own family line [מלבי״ם]. A more Midrashic approach suggests the ancient reference points directly to God, the Ancient One of the world, and the teachings of His Torah [רש״י, רד״ק].
The essence of this saying is that wickedness comes from the wicked, a statement that carries two main ideas. First, it speaks to how God directs justice in the world. According to the Torah, God brings punishment upon the guilty by using others who are also guilty. He arranges events so that a wicked person will ultimately fall into the hands of another wicked person. By quoting this, David hints that Saul will eventually face his downfall and be killed by wicked individuals [רש״י, רד״ק, מצודת דוד, מלבי״ם].
The second meaning focuses on human nature. A person's actions reveal their true character; someone who is wicked at heart will naturally commit wicked deeds. David explains that an innocent and upright person will not find themselves committing crimes. This is why he holds back from harming Saul. At the same time, David acknowledges the harsh reality that Saul possesses a cruel heart and would certainly not offer the same mercy if their situations were reversed [רד״ק, ביאור שטיינזלץ].
Based on these principles, David arrives at a firm conclusion. Whether Saul's ultimate punishment comes directly from Heaven or through the hands of another wicked person, David makes it completely clear that he will not be the instrument of that downfall. He resolves to keep his own hands clean and entirely free from sin [מלבי״ם, רד״ק].