A bullet from, say, a Kalashnikov rifle weighs only about eight grams, but leaves the gun at over 2,500 km/h. This is known as muzzle velocity; and it can easily kill.
Any bullet fired upwards slows down due to the Earth’s gravitational force. After reaching the maximum possible altitude, it turns and begins to fall down. Assuming there is no air resistance, the final impact speed of the bullet would be the same as at launch. In fact, however, air resistance cuts the impact speed to about 250 kilometers per hour. The bullet’s final kinetic energy is thus only ten percent of that at launch, but sill enough to cause death if it strikes someone’s head. If any other part of the body takes impact, a serious injury can not be ruled out.