If shot straight up it will return with dangerous but not necessarily lethal velocity. If shot in a parabolic arc it will return to Earth with lethal velocity.
In tumbled free fall it hits terminal velocity below lethality. If not tumbling, it’ll be fast enough to kill.
IMO, it will never return with the same velocity as when it left the muzzle if fired in ‘overhead’ fashion. It depends on the angle, which is what I’m taking from your explanation.
Right, a gun fired straight upward or close to straight upward probably isn’t going to kill anyone. But when it’s angled sideways that’s when things get dangerous. The horizontal component of the bullet’s velocity doesn’t have time to get reduced by air resistance to a point where it’s non-lethal.
Hatchers Notebook:
As I recall, if fired straight up, bullet (still spinning) will fall back, base first.