The responsibility lies with the nutbag who shot up the school and then committed suicide. It might be nice to try to find some reason or trigger for it, but really it was the actions of one individual performing a heinous act. He took advantage of the situation and executed his twisted plan, or reacted completely insanely, or something in between. While in a situation where he may not have had such easy access to such weapons (for whatever reason) it may not have happened the way it did, but there's no way of knowing for sure that he wouldn't have done something similar another way.
Whatever you think of her gun selection, in firepower or quantity, it seems she gained them legally and behaved with them responsibly. It can surely be argued that no matter how stringent any gun control law, short of completely banning gun ownership, she could have followed all of those more strict laws and still had weapons that her son would then acquire illegally and use incorrectly.
There's no evidence she didn't properly secure her firearms and that he simply defeated said security. Locking something in a gun safe isn't something that would stop a motivated and capable 20-year old. He could surely have known where she kept any locker or trigger-lock keys, and reached them. Short of that any number of tools could have been used to overcome many home gun lockers, especially those meant to keep children safe and not truly secure the weapons.
Some blurb I saw somewhere said that Connecticut only requires locking up firearms when there are minors living in the home, and since he was 20 he was not a minor. Yes, a responsible gun owner should have locked up the weapons regardless, but again, a 20-year old familiar enough with the weapons to do what he did would have surely been able to open said locks.
Additionally, while the actions show in hindsight that he was plenty unstable (tore up first-graders...'nuff said), there's not been convincing evidence presented (that I've seen or read) that indicates he was unstable to the point that one might think he would do what he did. Too many interviews point out "what a quiet person" or "nice fella" or whatever. I'm sure his mother thought she understood whatever was going through his head, as most parents will believe with their kids, even when they're wrong. We'd all like to believe that we'd be able to see the breakdown coming, and even if there was any indication he was about to snap, perhaps she didn't envision he'd snap like this, or on that day.
These are the basic facts. She had legally obtained weapons. He obtained them from her (and killed her with them). He is responsible for the actions that took the lives of those children and their protectors.