Because Johnny is a dunce, he had to shoot Jenson's hat off in order to recognise him. This made him vulnerable to attack, and he was tragically killed soon after when a Jackal Sniper scored a headshot on him.
Johnny accepted no blame for his death, and went on regardless. In fact, he showed no remorse at all, likewise to when Jenson's squadmates were also ruthlessly cut down by the overwhelming Covenant numbers. This is because Johnny did not have Johnny's Magical Assault Rifle with him at this point, he had to stick with the SMG and the Battle Rifle.
This meant that Johnny could show no emotion at all, and so he was impassive when his friends were killed. Despite this, he didn't seem that bothered even after he had retrieved his Assault Rifle.