The question “Why?” is part of what drives the human experience. Small children ask “Why?” (or “Why not?”) incessantly, and those with great minds — da Vinci, Newton, Einstein — ask “Why?” and “How?” about the fundamental nature of the universe and existence.

The rest of us are left asking why someone would kill 32 fellow travelers before turning against himself. The need for what we call “closure” is also an essential part of the human experience.

The simplest answer for many is to say that he was a “monster.” But what is a monster if not a human — a horribly flawed human? In Christianity, humanity was defined as flawed when it defied God, who cast it from the Garden of Eden. Literature exists to expose and describe the flaws of heroes and anti-heroes and those around them. Without flaws, we would not be human. With flaws, we should not be “monsters,” either.

The most rational answer seems to be mental illness. A person might kill someone in a fit of blind rage or jealousy or another strong emotion, when the primitive brain dominates. A person might plot against an individual or group who has, in their perception, wronged him or her, for example, a spouse’s lover. Such an action is not normal or justifiable, but it is not necessarily a sign of mental illness. Coldly gunning down dozens of random, innocent people in revenge for a lifetime of slights, perceived and real, after planning it and memorializing the thought behind that plan, seemingly without emotion, is.

What is “mental illness”? It’s too vague a term, as “physical illness” would be. In most cases, it’s relatively easy to test for and diagnose a wide range of physical abilities, ailments, and impairments — heart, lung, liver, kidney function, even brain function; the presence of antibodies; the malformation of organs; the deterioration of structures. I don’t think we have even begun to determine what the mental illnesses are, without which diagnosis and treatment are difficult and sometimes impossible.

Like physical illness, mental illness covers myriad disorders. Just as most people are physically ill at some point in their lives, even with a cold or flu, I suspect most people experience mental illness, too. I believe that many suffer from bouts of depression at least once in life, perhaps situational depression triggered by the loss of a spouse, other family member, friend, or a job, or depression that can accompany some illnesses, including those that are chronic. Bad things happen to nearly everyone, and only the most resilient individuals might never feel depressed for any length of time.

While many people have been depressed, mental illness carries a stigma because it is so often associated with the extremes — the cold-blooded sociopaths, the serial and mass killers, the murderous pedophiles.

We try to answer the question of “Why?” with various theories that it is too late to prove and that are now of no use. Sociopathy? Personality disorder? Depression? Which? Caused by what? Faulty biological wiring? Environmental and experience factors? Both?

Many children and even adults are bullies, more than many of us realize. My reaction to bullies, learned quickly at age 5, was to hold my head high and ignore them — that is, not to give them the reaction, thrill, and satisfaction they craved. I dreaded them and feared a surprise attack every day for many years, but found other things to take my mind off them most of the time. I may have thought of embarrassing them in revenge, but never of anything violent. I am not a bully.

We will never know how the man was bullied,or how much was real or perceived. A lonely, sick mind may make itself sicker with time, experience, brooding and the lack of balance provided by normal human interaction. In a nation as large as ours, in a world as populated as ours, there will always be sick minds, and there will always be bullies. They are both outliers of human society, and we have not learned how to manage them, or even to identify them. For now, the risk of a sick mind turning into the violent bully it most decries is one that we continue to live with. Let us hope that the lonely, the shy, and the depressed are not made to pay a price.

And let us hope that someday we are able to find out why.