Sometimes when a person uses just one word, you can tell a lot about them just because they used that one word. When I hear someone describing a bruise as a "hematoma" for instance, I reflexively think that person has had some medical training. Similarly, when I hear someone pronounce New Orleans as, "Nawlins", I assume they've spent some time near New Orleans.
And when I hear someone use the word, "evil" in an honest attempt to describe another human, I think that person is intellectually lazy and inconsiderate.
First of all, Evil doesn't really mean anything concrete, and regarding the laziness of the user, there are plenty of creative words way better than the uninspired, "evil": nefarious, dastardly, sociopathic, heinous, repugnant, malicious, and vicious to name a few. The main reason it's used is convenience, you know people know what it means, even if you aren't positive what it means.
If I tell you to think of what the word "angry" means, I bet you start to think up a definition, but if I were to ask you to tell me what evil is, I bet it's easier for you to give me an example than to define it. You might say the Sandy Hook shooter is evil, or Blair Witch is evil. But that wouldn't define evil any more than saying Ryan Secrest defines annoying. Evil seems easy to recognize, yet harder to define.
Evil is much more easily defined by what it lacks, than what it is. The popular accepted belief of evil is that it is synonymous with Godlessness. A quick search of various dictionaries uncovered several definitions peppered with synonyms implying Godlessness: wicked, sinful, sinister, depravity, esp. when regarded as a supernatural force. You can be a jerk or a low-life and still have some God in you, but if you're "evil", clearly, something supernatural is in play.
So now it seems like evil means 'the absence of Godliness', but defining something by what it lacks seems a little intellectually lazy, doesn't it? That'd be like if a 4-year old asked you what an ocean is and you said, "the opposite of a desert". So if we're going to define this word, we are going to have to state what it IS, then agree on that definition.
It turns out we are in luck and that science has already looked into this kind of thing! Who knew, right? Apparently using the word, "evil" to explain human behavior is a lot like using the words, "Santa Claus" to explain how presents wound up under a Christmas tree.
Now the psychological community and the DSM-IV don't use the word, "evil", but they do classify what we may think of as evil as someone who suffers from an Antisocial Personality Disorder. Referring to a murderer, not as evil, but as someone suffering from Antisocial Personality Disorder isn't just the intellectually correct way to phrase it, it's also the moral way to phrase it.
It's the morally superior option because it allows us to hate the sin yet love the sinner, as difficult as that may be. It's constructive also because it helps us to understand this wasn't a blood-thirsty savage acting in accordance with some evil deity with horns and a trident. Instead this was someone who wasn't in control of their unfortunate mental state and is worthy of some pity. Had we been unlucky enough to share in their Antisocial Personality Disorder, we'd probably do similarly "evil" acts as well.
To brand someone as evil is the opposite of compassion and is a particularly dangerous logical flaw because it implies the judger is somehow closer to God. When you label someone as evil you aren't saying, "Unfortunately, you were randomly cursed by a debilitating psychological malady", but instead says, "God is in me, but not in you. I'm good and you are evil.". It's dangerous because ever war in history has started with the pretense that one side's lives were less important than the other side's.
I'm as saddened and outraged by the Sandy Hook tragedy as the next guy, and I can think of several places for blame. But it's dangerously myopic to think the cause of this shooting relates to the amount of God that was or was not in the shooter.