The primary cause was their severe mental illness, and likely, that most of them either weren't taking their meds properly, they weren't prescribed properly, and/or they weren't receiving complete, proper treatment. Psycotropic drugs help far more people than they harm, just as firearms prevent far more crime than they cause. I am by no means excusing his actions, but Alexis reached out for help many times, to the VA, the RI cops and others, and he didn't receive the help he needed. For all of the reasons Ms. Coulter stated. They system let him down, and ultimately let the victims down, just as it has done so many times in the past.
Fixing the system is a slippery slope though. On one hand, we need to get some of these people off of the streets. On the other, therapists and psychiatrists tend to lean left, and they might be a little liberal (pun intended) in their decisions to declare someone dangerous and therefore deny them their 2nd Amendment rights. Somewhere there is a workable compromise.