If you Google "Massacres in the US" you will see that there have been many, dating back to the country's beginnings. Interestingly, there seem to have been relatively few in the 1950's-70's when many of us came of age, perhaps adding to our perception that they are something new.
Recent ones seem to be more often due to a random crazy, rather than some political or criminal event.
One thing that is different now is the ready availability of devastating weaponry. When else in history has it been feasible to arm yourself to the teeth, protect yourself with body armor, booby-trap your apartment, and shoot 70 people in a minute or two? I am not trying to start a second amendment debate; just saying that the ability for a crazy person to do a lot of harm is now dramatically enhanced.
