There may be people above and/or below you, to either side, and maybe behind.
In most places, keeping the gun pointed more or less straight up or down is a reasonably safe direction. Not always the case in apartments.
Firing through the floor and hitting the neighbor below . . .
Besides risking someone's life and proving what an imbecile you are, it gives the antis another chance to say, "SEE? Guns are an accident waiting to happen!"