There have been mass killing with knives. One of the worst US mass killing was by arson. The worst was by using box cutters to hijack air planes and crash them into buildings.

The problem is not the weapon. Semiautomatic weapons have been with us in significant numbers for over a century.

The problem is with people.

Furthermore, mass killings are not new. They appear to increase for two reasons: instant media completely biased in favor of restricting gun rights; and, a redefinition recently of "mass killings" to cover smaller numbers of people, and to cover gang gunfights, which kill far more people than the well publicized "mass killings."