I found this to be a fair and well-written piece on a serious, tragic problem and it's complicated and tricky solution(s).
https://fivethirtyeight.com/features/mass-shootings-are-a-bad-way-to-understand-gun-violence/
Circumstances such as this one are unfortunate, because (as someone mentioned earlier) it sounds like the guy just cracked. No warnings, no obvious patterns. Further, this is a difficult issue because it's about the constitution - air travel, drivers licenses, health care, even marriage or abortion can be interpreted as being protected under the constitution, but firearms are deliberately and specifically mentioned. People often say "the second amendment is about muskets, not assault rifles." That's absurd. If that argument holds merit, then the first amendment is about quills and pressed newspapers and town criers, not the internet or television or kneeling at football games. The patriots had just finished fighting a war for "freedom" against their own government, and wanted to make sure subsequent citizens would have the resources necessary to maintain that "freedom." It is the supreme law of this land - literally. Rather than blame guns, the NRA, gun-toting republicans, or elected officials, we should lobby to amend the second amendment. Guns or insufficient laws are not the argument, the constitution is.