The issue is you can revise all you like, these kinds of killings won't stop.
Gun laws have little to do with this from a security standpoint because he was using firearms that were either illegal (homemade) or he worked like hell for years to get and are almost impossible.
You can ban every weapon in the US from being sold tomorrow, there's still 350,000,000 on the open market, not including homemade weapons. Then theres another 100,000,000 coming across an open boarder because we've created an arms race for guns. Oh, then there's the criminals who will be making them out of their garage. Believe it or not, some of the deadliest weapons are extremely easy to make. So that simply won't work.
Honestly, if you can't catch it before hand and spot it then you're probably gonna lose. Maybe have employees go through mandatory training on signs to look for, and possibly give the states and local authorities more funding for mental health. Other than that I just don't think there's many answers.