It's their job, I'm sure we all realize that. After the commission of a crime, it's the function of law enforcement to look for an understandable explanation of why the crime occurred in the first place. However, the more heinous the crime, the less likely there is to be any explanation. In the case cited in my title for this post, and any other case where people kill other people that they don't even know, and who presented no perceived threat to them, there are only two possible motivations:
1. Mental illness, or
2. the doer is the personification of evil
Mental illness - as a concept - is understood by most lay persons. What is not totally understood is the root causes of many kinds of mental illness. Most mental distress isn't the result of some physical illness or bacterial infection. It stems from a series of perceived wrongs or inequities, and their accumulation and storage. Frequently that is the result of chemical imbalances in the brain. The inability to discard such perceptions, after objectively evaluating and processing them, eventually creates more psychological "weight" than the individual can bear, resulting in mental collapse.
"Objectivity" is skewed in the mentally ill, and therefore perceptions are distorted, which results in skewed objectivity, which distorts perceptions, ad infinitum. We could easily get hung up on the "chicken-egg" argument here, but it really makes no difference in the outcome. We must also understand that perception creates our "personal reality." When several people witness the same event, they each process it in their own fashion and may perceive it as totally different events. Some processing is intellectual, but all processing is colored by cultural, emotional, political and spiritual influences. Thus is our personal reality formed.
An example of this would be the so-called "Extreme Fighting" matches. To most people from civilized backgrounds these physical confrontations - for the sake of feeding egos - are seen as barbarism. Two people enter a cage and proceed to beat one another senseless (my assumption here is that they had some sense to begin with). These matches always result in the victor injuring, sometimes in maiming, and, on rare occasions, killing their opponent. There are those others who see the same event as entertainment, fun, a test of skills, heroic personal combat, etc..
The key words in the preceding two paragraphs are "perceived" and "accumulation and storage". Most people are capable of observing, processing, evaluating, and then compartmentalizing events. Some compartments are for the purpose of recall at a later time, while others are for discards - things that the person has witnessed that are either neutral influences, or negative influences. These are events that have little or no positive influence on their life, and are otherwise just burdensome.
There is disagreement within the medical and psychological communities over the possibility of somebody being evil by nature. Arguments against an "evil nature" would fall short of explaining the "motives" of people like Adolf Hitler, Idi Amin, Papa Doc Duvalier, Ted Bundy, John Wayne Gacey, Richard Speck, the 9-11 terrorists, et al.
The motives behind the actions of others are not always visible to others, simply because others do not share the actors' personal reality - it's as foreign to them as Egyptian hieroglyphics would be to the vast majority of the worlds' people. Searching for motives created by a deranged mind, beyond the elimination of what the "normal mind" might see as motive, is generally a waste of time, effort and resources.
The willful taking of a human life, one that is otherwise unknown to you and presents no apparent threat to your life, is either an act of severe mental illness ... or evil personified.
I could be wrong about this, I suppose ... but you'd have to be insane to think so!