It's not one or the other and we obviously need to improve social circumstances for people who tend to get influenced and are in a vulnerable mindset, but at the same time tightening access to weapons will only improve the situation, and in my opinion on a considerable level. There's no denying the fact that having over the counter weapons is the biggest factor in allowing literally anyone to turn into a mass killer overnight. Like I said crime will not end with it but it will be a lot harder for anyone to actually act on his emotions/whatever on a whim.
At the end of the day it is the same situation as with global terrorists, except in this case the culprits are inside the US where they can be governed into not being able to implement any harm onto others.