The barrier means nothing since the devil king is already dead. Also I bet they will try to unleash an attack big enough to wipe out everyone, which means it could hit mc and that causes them to die.
It would be a good bit to have the demon get the barrier dispelled, laugh about how everyone's doomed now, and then see the devil king just flop out already dead.
This manga is way too cringy for me. Even Saitama punches bad guys, and he's partially a mocking take on overpowered anime characters. This guy just wishes they were dead, and that's it.
Oh look, more soon to be corpses. I wonder...since the MC has total control over what death is, can he counteract the other dude's instant death magic?