Sometimes, armed citizens in the US stop or mitigate this kind of senseless violence. In 1997, how many of you remember the Mississippi school shooting? More of us would if it had not been for the armed school principal.
From wiki:
The incident began on the morning of October 1, 1997 when Luke Woodham fatally stabbed and bludgeoned his mother, Mary Woodham, as she prepared for a morning jog. At his trial, Woodham claimed that he could not remember killing his mother.Woodham drove his mother's car to Pearl High School. Wearing an orange jumpsuit and a trenchcoat,[1] he made no attempt to hide his rifle. When he entered the school, he fatally shot Lydia Kaye Dew and Christina Menefee, his former girlfriend. Pearl High School assistant band director, Jeff Cannon, was standing five feet away from Dew when she was fatally shot. Woodham went on to wound seven others before leaving, intending to drive off campus and conduct another shooting at the nearby Pearl Junior High School. However, assistant principal Joel Myrick had retrieved a .45 pistol from the glove compartment of his truck and subdued Woodham inside his mother's car. Then Myrick demanded "Why did you shoot my kids?" to which Woodham replied, "Life has wronged me, sir."[2]