In the United States there have always been killings, atacks and more, it is something that is seen every day.
The worst of all is that on many occasions they are young people who have isolated psychological problems and decide to attack others.
The ones that have claimed the most lives are wars and terrorist attacks.
I would say that the biggest attack in the USA was the terrorist attack on September 11, 2001 in the world trade center in NY
2749 people died.