I'm still waiting for Vanya to divulge the one common trait among most mass shooters.....
I personally believe every situation is different, and most likely there is a myriad of "reasons" for these tragic events occurring, with no "universal" common denominator. Is one factor frequently present? Probably so. Does that necessarily mean it is always the true culprit? Not likely. I would postulate that in most situations, several or even numerous factors combine to produce the end result. The permutations are endless, which makes these tragedies so difficult to predict and prevent.
However, focusing on what we do know, it is plain that humans killing each other is not a new phenomena. Since time immemorial, this has occurred. So I think it can be safely assumed that there exists within all of us the "capacity" to do such things. That is not a thought that makes us comfortable, but it is there.
Obviously, the vast majority of humans never access that capacity, so what is it that separates the majority of us who live peacably from those who commit atrocities?
There must be some code, some modicum of restraint that helps us not cross that line. Could it include things such as:
Fear of punishent? Respect for human life? Love of your fellow man? Respect for and adherence to the law? Spiritual desire to adhere to basic tenets of faith?
Conversely, there must be some trigger, some condition, some convergence of factors, that breaks through the restraints and allows individuals to access that capacity. This is why we endlessly search for WHY? when something like Columbine or Sandy Hook occurs.
The simplistic and rational side of us would say, "Let's just find the cause, and eradicate it." Or, if we can't do that, let's put in place safety processes that will prevent a line-crosser" from ultimately carrying out his/her carnage.
Unfortunately, neither is totally possible. While we may make strides in both areas, the sad reality is that the heart of man is inherently evil, and we do not live in a perfect world. Try as our society might to make us all as safe and risk-free as possible, it is a fool's errand to take any assurance that the various laws and societal norms will keep us safe.
It boils down to the individual. We must all take responsibility for ourselves first, and then for those around us whom we perceive could possibly be susceptible or prone to the identified risk factors.
And, we must avoid the trap of saying, "It couldn't happen here", or "I just don't know what I can do."
Finally, we must not retreat from the obligation we have to protect ourselves and our loved ones. As an Illinois resident, I am keenly aware of the lack of our state's support in allowing individuals to protect themselves, by denying us the right to conceal carry. However, that appears to be changing, and ultimately, we will have a very important tool in helping us in the area of personal safety.