Gunshots echoed in Yuki's ears constantly as a grim reminder of his position as the first diary owner. Even when he wasn't at a battleground, struggling to avoid certain death with Yuno at his side (acting as the brawn to Yuki's not-quite brains), he still heard gunshots. His heart skipped beast at the mention of guns.
In Yuki's dreams, he always held a gun that never worked. Instead, the gun would bleed through his fingers; or worse, it would explode in his face.
His worst enemy was his mind; Yuki would forever remain a prisoner.