"Can't blame this one on prior-diagnosed mental disease or defect."

No, but there's are a non-trivial number(estimates I've seen are anywhere from one to five percent) of the populace who are psychopaths and some non-trivial percentage of those are undiagnosed. What percentage of those will turn violent? Who knows. This guy had a father who was a diagnosed(?)  psychopath, so a genetic pass along is very possible. 

Easy access to firearms+accessories that allow a single civilian to cause a full battalion of casualties in ten minutes is at the root of this horror. Trying to explain it away in any other way is simply an attempt to deny reality.