it was used by a mentally ill person to kill children
that's how it was designed?
Like I said, the best you can claim is that it wasn't a design consideration. Which is likely true, but irrelevant. If it had been a design consideration, and the result is still a device that can be used by children and the mentally ill and orangutans, that would be a design flaw and the manufacturers would be potentially liable.
Back in the day, cars were designed to favor sex appeal, and survival in an accident wasn't a design consideration. Seat belts and airbags weren't required, and they generally weren't equipped either. The government mandated that cars design for accident survival, and instituted formal testing standards as well as specific measures, and cars became a lot safer, but it wasn't sufficient for car makers to simply say that their products just weren't designed to experience accidents because accidents happen and are reasonably foreseeable.