3 of the worst ever mass murders in America didn't even facilitate a gun:
Worst School Massacre in US history: Bath, Michigan School Massacre. 1927. Murder accomplished with explosives. 44 victims (equal to the Columbine and Virginia Tech massacres combined).
Worst Domestic Terrorist Attack in US History: Oklahoma City, Oklahoma, Alfred P. Murrah Federal Building bombing. 4/19/95. Murder accomplished with a rental truck full of fertilizer based explosives. 168 dead (including many children in an on-site daycare).
Worst Foreign-based Terrorist Attack in US History: September 11, 2001, attacks on NYC, PA, Pentagon. Murder accomplished with box cutters and commercial airliners. ~3,000 people dead.
There are mass murders all over the world without the use of guns. The worst by far is this murder by fire that killed 198.
Daegu subway fire, Daegu, South Korea: 2/18/2003. 198 killed, 147 injured. A 56-year-old unemployed taxi driver, dissatisfied with his medical treatment, sets fire to a crowded train.
Arson, Stabbing Rampage in Seoul South Korea: 10/20/2008. 6 people dead, 5 from stabbing. 7 others wounded, 4 seriously. An angry man felt people “looked down on him.”
Anti-police stabbing spree in Shanghai, China: 7/2008. 6 Police Officers stabbed to death, 4 wounded. 28-year-old man angry at police attacked a police station with a knife.
Akihabara Massacre, Chiyoda City, Tokyo, Japan: 6/8/2008. 7 people killed (3 struck by car, 4 by stabbing), many more injured. Man slammed into a crowd with his car, then jumped out and began stabbing people to death.
18 year old slashes 4 to death in Sitka, Alaska, US: 3/25/2008. 4 people killed. 18-year old (old enough to purchase a rifle over the counter) kills 4 people, related to him, with a 5-inch knife.
Stabbing Spree kills 2, Tsuchiura, Japan: 3/23/2008. 2 killed, 7 wounded. Man “just wanted to kill anyone.”
Stabbing spree wounds 41, 6 seriously in Berlin Train Station:5/26/2006. 41 wounded, 6 seriously. Thankfully no one died in this attack, but not for lack of trying on the part of the drunk 16 year old.
4 killed in stabbing spree in London, UK: 9/2004. 4 killed, 2 wounded. Mentally ill man attacks mostly older people.
6 killed over Xbox dispute in Deltona, Florida, US: 8/6/2004. 6 killed. 4 men (all old enough to legally purchase firearms) bludgeon 6 people to death with baseball bats over purloined Xbox.
Osaka School Massacre, Osaka Japan: 6/8/2001. 8 children dead, 13 other children and 2 teachers wounded. Committed by 37 year old former janitor armed with a kitchen knife.