In 1883, it was a rope and a tree. After the accused received a "speedy and fair trial", the legal system hung them by by the neck until they were dead in public. Not only was it humiliating for the criminal, it served as a deterrent for others to not kill innocents. Make their family watch them hang too. Make it so horrible that nobody dare shoot up a school. Make sure there is lots of media coverage so anybody can see a cold blooded murderer die.