שמואל ב, פרק י״ח, פסוק כ״ז

As a runner approaches the city, his identity is revealed long before his face can be seen. The watchman observing from afar recognizes the approaching figure simply by his unique style of running. The distinct pace and manner of movement perfectly match those of Ahimaaz the son of Zadok, allowing the watchman to confidently identify him [מצודת דוד, אברבנאל, ביאור שטיינזלץ].

Upon hearing who is running toward them, King David immediately deduces the nature of the news based entirely on the messenger's character. Recognizing Ahimaaz as a deeply good and complete man, the king understands that his arrival cannot possibly be a retreat from the battlefield. A man of such solid character would never abandon the front lines; therefore, his journey is not an escape, but an official mission [אברבנאל].

Beyond simply ruling out a cowardly flight, the king confidently anticipates a positive update. It is the natural inclination of a goodhearted person to eagerly desire to deliver good news [מצודת דוד]. Because of this inherent trait, King David is certain that Ahimaaz is running with the explicit purpose of sharing a positive message, arriving specifically to carry glad tidings [ביאור שטיינזלץ, רד״ק].