Again, the only four proposals I could see that could've potentially prevented this tragedy are:
1. A blanket ban of AR-15s or semi-automatic rifles in general... which even then is sketchy at best in regards to whether or not it would actually prevent this (he could always have gotten... a different gun, or illegally obtained it), it would not be remotely feasible to implement, I doubt the government would even be able to confiscate 50% of those guns, and really just ethically a serious infringement on people's second amendment rights
2. A blanket ban of guns... which is completely against the second amendment and people's right to protect their lives and property and a curtailment of their liberty, would be horribly ineffective at implementing, etc.
3. A change in the way mental illness is classified... the dude did pass the background check after all.
4. Heightened security at schools.
The first of these two options are unpalatable due to both the inefficacy of implementing them and the ethical dilemma with curtailing people's liberty and property rights, and the last two of these four options are not changes to gun policy.