Guns are just immensely more suitable for a mass killing spree than other readily-available weapons.
Fertilizer and kerosene are readily available, and suitable for a mass killing spree.
5.56mm
“Fertilizer and kerosene are readily available, and suitable for a mass killing spree.”
Not really. You can mix up all the fertilizer and fuel oil in the world, it won’t do anything but burn on its own. You need a not readily available high order explosive in order to create a shockwave to detonate that mixture.