The saying ‘What goes up must come down’ is an appropriate starting point.
If you fire a gun into the air, the bullet will travel up over a kilometre high (depending on the angle of the shot and the power of the gun).
— Yusuf Abramjee (@Abramjee) April 21, 2018
Once it reaches its apogee, the bullet will fall. Air resistance limits its speed, but bullets are designed to be fairly aerodynamic, so the speed is still quite lethal if the bullet happens to hit someone.
In depopulated rural areas, the chance of hitting someone is remote because the number of people is low. In crowded cities, however, the probability rises dramatically, and people get killed quite often by stray bullets.