It is always horrible and incredible to hear of such disasters.Why do you think these killings frequently occur ?
Considering the cause and numerical potential, these tragedies are really not all that frequent.
There is always some kind of emotionally energized self-image payoff and the existing mindset contains insufficient societal norm mores to prevent it and/or contains concepts that support it. Then there is a related trigger.
For Mateen, it was to avoid Muslim fundamentalist Daddy's disapproval and to maintain Daddy's approval, the emotional energy component. Omar Mateen's mindset contained Muslim fundamentalist concepts of killing infidels augmented by a similar sentence for gays, the mindset that set the stage for the self-image payoff as a "great Muslim". The triggering mechanism was seeing two infidel gay men kissing in front of his son, and, as a dad, he internalized what his own dad would say needed to be done .. and he did it.
Mateen might not have failed a sanity test; his cognitive/emotional state might not have been pathological, even if he behaved "like" a sociopath.
The particular emotional energy, cognitive concept, and self-image payoff combination was sufficiently set, triggered into action by the observed "infidel"-gay event.
Sandy Hook, Oklahoma City, San Bernardino .. all these perpetrators had the same emotional-energy, mindset concepts, and self-image payoff formula. Then a specific trigger-event occurred .. and the rest is horrific history. They differed from Orlando only in specifics, such as who the family-of-origin person(s) were, whether it was idealization or contempt regarding them, how the mindset concepts were implanted and what they were, what the self-image payoff was, and what triggered the event into action.
This is what makes some forms of extremism so scary. A great many people have the family-of-origin damage set. They're ripe for idealization and contempt extremes in concepts to settle in their "fertile" minds. Unresolved family-of-origin damage comes complete with attendant low self-image. Who knows what the trigger events will be.
The extreme concept component is why we intuitively find religious or political extremes worrisome. All it takes is some imam, preacher .. or politico .. to plant extremes in "prepared" soil.
Whether the tool might be an assault rifle, fertilizer, hand weapons .. or a backpack-size nuke .. it's still all bad.
The easiest prevention is take away all the weapons (still an essentially impossible task), and abolish dangerous concepts (again, impossible to do). So not so "easy" after all.
Ultimately, healthy childhoods prevent dysfunction-based idealization/contempt from the mind .. but that too is impossible to create world-wide. Best we can do is help parents raise healthy children here and now.
So we are left here in America with a rather difficult and not very effective task: keep out illegal aliens and allow legal entry only after a person is scrutinized well. Even if we succeed here, it's effect is like shooting an elephant with a BB gun.
Ultimately, all this tragedy goes away when there is far more basic needs resources available worldwide than demand for them. Today, we have simply the opposite. Parents struggling in need, unable to feed and protect their children, to whatever degree, sets the stage for dysfunctional parent-child relationships ... . We may a century or so away from this, and then again providing only that we really, really, work at it to create a significantly negative population change rate, first in America, and throughout the rest of the world.