Megan McCardle provided a prime example, as linked to by Michael, of one particular take on what transpired in Newtown as well as other mass shooting that tends to get in my craw [emphasis mine]:

Most crimes are motivated by unlovely impulses that are at least comprehensible: the desire for money, sex, respect, revenge.  We don't do these things...But we can understand why people want to--we know what someone is after when they hold up a liquor store, or even kills their spouse for the insurance money.  Understanding is not sanction: these crimes still have the power to anger and horrify.  But they're comprehensible, and that comprehensibility is surprisingly comforting.  

The alternative is Newtown.  When one tries to picture the mind that plans it, one quickly comes to a dead end....Trying to climb this mountain of wickedness is like trying to climb a glass wall with your bare hands. What happened there is pure evil, and evil, unlike common badness, gives an ordinary mind no foothold.

Since we can't understand it, we can't change it.  And since we can't change it, our best hope is to box it in.

The notion of evil when it is used to describe such heinous acts is usually not describing it as something morally wrong or something that is a cause or source of suffering, injury, or destruction.  Instead, it is used to describe the acts arising from an evil force, power, or personification, in other words, arising from the actions something non-human or un-human.

When McCardle calls Lanza "pure evil," she is removing him and his actions from the realm of human potential.  It is like walking outside into a snow storm in early summer and saying it is "unnatural." Whether we like it or not, Lanza is human and his behavior is within the range of what it means to be human, just as freak June snow storm, while uncommon, is still very much a part of the natural spectrum of weather conditions.

There is a sort of "natural" reaction to not want to associate one's self as being part of the same team or tribe as someone like Lanza.  "He's not one of us."  But it goes deeper than this, I believe.  To acknowledge that Lanza is one of us to acknowledge that the heinous act he commited is somehow, somewhere within us if our nuture/nature circumstances were somehow just a little different.

Hitler and the Nazis become monsters because we don't want to believe our society could commit such acts as the holocaust if faced with similar circumstances. 

And this is why accepting Lanza as human is  important.  This is why it's a big deal if someone wants to call Lanza pure evil or the Nazis monsters.

In McCardle's world, she can comprehend the spouse who shoots someone for the insurance money or, I would guess, the lover who has cheated with another. Lanza had a reason for what he did.  Not knowing exactly what those reason are does not make them incomprehensible, just not known. 

One theory thrown out there is that the age of the kids was about the time Lanza started to get bullied for being different.  It is comprehensible, which is as McCradle points out not the same as sanctioning, that Lanza was seeking some kind of revenge, and some kind psychological closure. 

I worked with schizophrenics for a number of years and during that time all of the "bizarre" behavior and statements I witnessed always made sense once one understood the information the individual believed they were receiving about the world around them and their place in it.  If one believes that another is trying to steal one's soul while one sleeps at night, one might go to some extreme measures to prevent that.  (I would note that in those years at a facility with over 70 schizophrenics living under the same roof, there was actually very little actual violence).

If you see someone whacking a piece of rope on the ground, you might think he or she is being bizarre, that is until you know that they had stepped on the piece of rope and believed it to be a snake.

As humans, just as any living creature, we act with purpose, and we act to deal with the world as best as we can.  Because of language, that purpose and our understanding of what the best course of action can be can become twisted.  We can be destructive, we can act in ways that we as a society is morally and ethically wrong.  When the "wires in our brains" get crossed, we can look to the outside world as bizarre and even non-human.

There was a time when the schizophrenics were locked away in the most horrendous conditions because those at the time did not understand their suffering and plight.

If we believe the Nazis are monsters, we are more likely to repeat history.

If we turn those like Lanza into pure evil, we are less likely able to develop the support and interventions necessary to facilitate the decrease of those who end up believing creating such scenes of death and grief as means to an end.  We end up saying like McCardle does: Since we can't understand it, we can't change it.

But we can understand it.  We may not like it.  It may make us sick to the stomach.  But we can understand it.  If we face it as one of the faces of humanity.