The Boston Marathon bombing follows mass killings in the past year in an Aurora, Colo., theater, at a Sikh temple in Wisconsin and at Sandy Hook Elementary School in Newtown, Conn. The body count might have risen sharply if the bombs had been of dynamite like what killed four little girls in 1963 at a Birmingham church, or of the fertilizer type that killed 168 people and destroyed the Murrah Federal Building in Oklahoma City in 1995, instead of relatively low-tech gunpowder bombs made by amateur bombers.