With anything that is manufactured, there is intended use, and then there is use that falls outside of the intended use or manufacturers control. Firearms are of course designed to inflict harm on humans. But they are designed to do this in either combat situations, or defensive situations only. Gun designers are not evil people that are designing firearms to be used in tragic mass shootings. In the same way that hammer designers are not designing their hammers to be used to hit people, or car manufactures are designing cars to run people over. Are all aware that these things can happen? Of course. But if you want to start holding designers and manufacturers culpable for things that happen with their products that are obviously outside of their intended use, notbjng would ever be made.
This, as with all of these types of tragedies, is a matter of personal responsibility alone. There could be a billion AR15s in circulation, if this kid had decided not to commit this crime, it wouldn't have happened. Guns aren't walking them selves into schools and killing people on their own.