I agree with you to a certain extent regarding the mom's accountability. In my moralistic mind, I always feel the parents are responsible for their children, no matter how old they are. But that is only in my mind.
However, the fact that NL taught her unstable son to shoot and then went on vacation without a caretaker to watch over him, and might or might not have left her gun safe unlocked also attests to how much she trusted him.
I think she was mistaken to trust him, given his issues. But she was his mom, and I suppose she wanted to give him the unconditional love she thought he needed because of his challenges. Sometimes parents have the best intentions, but the consequences turn out wrong anyway, as it did in AL's case.
In the end, the executions were committed by AL alone, and he was an adult who evidently knew the difference between right and wrong. So the culpability of the mass shootings is AL's alone. I think until there is proven evidence of NL's negligence in leaving her gun safe unlocked, it's unfair to condemn her.