Conservatives On Evil
Evil is a concept largely reserved for the religious and for conservatives, who as a result don't need to ask "Why does somone do something bad?"
The answer is obvious, indeed, almost tautological: "People do bad things because they are evil."
Implicitly, however, this view also sees evil people as rational actors who responds only to externally imposed brute force. A more nuanced recognize that some people are more evil than others, but retains the concept.
This worldview also justifies punishment and retribution. If someone is evil, and it is by definition good and right to punish them for their evil acts. And, if someone is completely evil, it is good and right to end their existence, even when one is acting in cold blood in the court system, rather than in the hot blood of a situation where one must use violence to stop violence.
The trouble is that approaches to crime control which are rooted in this world view don't work.
Liberal On Evil
Liberals are more skeptical of the concept of evil and so they are more prone to asking "why do people do bad things?", with an intent to get some sort of meaningful answer not implicit in the question itself.
Progress Now today highlights a recent book, "The Lucifer Effect: Understanding How Good People Turn Evil" by Philip Zimbardo, that focuses on one class of bad actors, sociopaths and their less extreme cousins, normal people who behave badly as a consequence of their socially assigned roles.
A sociopath isn't mentally ill. Instead, a sociopath is someone who as a result of their social situation (the classic example is gang members) develops a value system that tolerates and even glorifies anti-social actions.
Philip Zimbardo, along with Stanley Milgram, are famous for their experiments that show that it is actually exceedingly easy to turn ordinary nice people into people who violently abuse their power, even in a quite artificial settings on a college campus. This isn't a phenomena limits to those who spend a lifetime in a horrible situation. Zimbardo's book talks about how how context can turn people into monsters.
The book is hauntingly current in the wake of the Abu Grahib scandal in which American prison guards abused Iraqi detainees in weird sexual and sadistic ways.
In the same vein, we increasingly are learning that many suicide bombers are more well educated, middle class, patriotic people who are willing to sacrifice for a believed benefit to their peers, rather than what most people historically have believed, which is that most sucide bombers are deranged or stupid, a lot of terrorism starts looking like it falls into the realm of acts of war driven by one's social and political perspectives (similar to lawful use of violence by soldiers and law enforcement, but influenced by different, but mainstream values about the propriety of targetting civilians) rather than like crime.
Recent statistics showing large numbers of Colorado prison inmates are gang members, and events like yesterday's major bust of a crips affliated gang in metro Denver, suggest that this is one important part of criminal activity.
Another cause of bad behavior is psychopathy. While not an official psychiatric diagnosis at this time, researchers in the field have identified a mental illness which is firmly entrenched at a young age which comes close to the folk definition of evil. Psychopathy doesn't necessarily lead to evil behavior, but is found in a large share of serial killers and con men. Psychopathy doesn't inherently imply that someone will be violent, does usually does imply that their conscience will not restain them from acting on their impulses, that they will go to great lengths to fight boredom, and that punishment is often ineffective in causing someone suffering from it to change their behavior.
Less frightening cousins of psychopathy, so far as I can tell, as they also involve impulse control issues and difficulty dealing with boredom are a genetically linked personality trait known as novelty-seeking (a trait associated with luring sufferers into criminal activity), and attention deficit disorder, which is also believed to have a biological basis, and is like the other two associated with insufficient response to dopamine and impulse control, but generally far less powerfully, although it is also believed to be corollated in a causal fashion with delinquency.
I suspect (but lack the expertise to state definitively) that at some point psychopathy, a novelty-seeking personality and ADD will be grouped together as related conditions, much as various autism spectrum disorders are today.
One of the big picture parts of both the mental health and physical health research agendas is to reach a point where more diseases can be categorized by cause, e.g. autoimmune disorders or dopamine processing disorders, which suggest treatments, rather than in the traditional symptomatic method. My guess may be wrong, but the larger project is real and valuable and regularly making surprising connections. Who knew, for example, that many forms of diabetes have a central nervous system auto-immune component, just as M.S. does?
Mental health conditions associated with impulse control and boredom, of course, are not the only mental conditions associated with crime and bad acts. Historically, many cases involving the insanity defense in criminal law (and many of the inquiries into testamentary and contractual capacity) have focused on mental health conditions that give rise to delusions that make it impossible for the actor to know what he is doing or to understand the rightness or wrongness of what he is doing. A recent appeal of a Colorado criminal case drew the line, however, at allowing bona fide hallucinations induced by voluntary drug use to provide mitigation or justification for an individual's acts, an LSD trip and a schitzophrenic delusion weren't morally equivalent in the eyes of the court.
The kind of mental health condition that gives rise to incidents like the Virginia Tech massacre, and these incidents do seem to follow a certain profile, is different again. The perpetrators don't seem to fit the profile of the cold, consciousless individuals who go onto become serial killers. They do seem to be loners, with life long struggles with mental illness of some kind. But, I'm not enough of a psychiatrist to tell you precisely what to call the one or several types of mental health issues are associated with this kind of incident.
While some psychiatrists believe that aggressiveness is another trait which is both linked to crime and may have some biological as well as social components, aggressiveness itself, while linked to bad acts, is not very closely linked to the kind of bad acts we customarily think of as evil. Spur of the moment bar fights, for example, can kill, but are rarely considered as culpable as a pre-meditated murder.
There are probably others I've missed -- the apparently obessive-compulsive personality disorder, for example, that gives rise to the cleptomaniac. Another major root cause, that lies somewhere between being a biologically based mental health issue and a psychological problem that is not immutably part of a person, is substance abuse and addiction.
Crime Committed By Rational Actors
But, it is also worth noting that bad, or at least, illegal behavior, doesn't need to imply either a mental health issue, or a deeply warped set of values. Sometimes, people do bad things, or commit crimes, simply because they are "rational actors" or people who more or less accidentally slip over a line from accepted to impermissible behavior. The line between a lot of business fraud, and the ordinary and accepted commercial practice of puffing in business negotiations, for example, can be a thin one. The classic example of the rational actor criminal is the man who steals bread because he is hungry. Someone might intentionally omit income from their tax return because they don't think that they will get caught, feel that no one will be hurt if they do, and feel that they need the money to provide for their family. Less dramatically, almost everybody intentionally drives over the speed limit.
Not all bad acts are evil acts, utterly devoid of justification or excuse. And, the fact that we know what motivates a bad act doesn't necessarily, or even often, mean that we should refrain from punishing it. Similarly, mental illness does not itself make commission of a crime inevitable (or even likely, even if crime is more likely for one person than for the average person), it simply makes it more challenging for some people than for others to resist committing one. There is considerable academic literature on how and to what extent mental health can predict violent recidivism.
But, to effectively prevent crime, you have to understand its root causes, and you must further understand the relative importance of those root causes. It is plausible to believe that a lot of people who commit economically motivated crimes driven by poverty and an inability to earn money by legitimate means. Prostitutes, thieves, smugglers and drug dealers, for example, may be basically rational actors who would are in hard straights. A lot of civil rights and human rights violations, fraternity hazing incidents, and gang related violence, may be more easily chalked up to sociopathic causes. Many of the most gruesome, headline making offenses, and similarly, many of the most odd, almost benign, motiveless crimes, may be attributable to mental illness.
Measures designed to stop the next Virginia Tech may be worthless, or even counterproductive, at controlling drug dealing. But, until a better model of why crimes and bad (even evil) acts are committed becomes more widespread, the prospects for progress in reducing the incidence of these acts seems dim.