The person wielding the weapon (of any kind) is at fault. Not the tool.
We need to know if he obtained the weapon legally? Was he competent? (His brother speaks of mental illness in which case he should not have received a gun permit.....if indeed he had one). Perhaps his education (in public schools?) taught him something about not respecting a person's individual right to life liberty and the pursuit of happiness.?
A gun just lying there on its own does not kill.