If a bullet is fired into the air and then falls back down to the earth from the sky and it hits you in the head then yes it could potentially kill you.
The higher up the bullet when it falls down from the sky the faster it will fall and then it can penetrate your body and head and kill you that way.
So it's very dangerous to shoot a gun up into the air because what goes up does come down eventually and it could potentially kill someone especially if it penetrates a persons head and brain.
At the very least if the person gets hit in the head with a bullet from the sky then it could lead to brain damage if it doesn't cause death.