If you murder someone...not kill out of self-defense, or kill as part of a job (whether you think that's a legitimate job or not is unimportant) or kill accidentally...but deliberately take a life, there is something wrong in your brain. There are people, right now, who are using that fact to try to either promote or attack gun control, when the answer to gun violence will always require both mental health services and sane gun laws.
There is a legitimate concern that when we state the above-that murdering someone is diagnostic criteria, in and of itself, for a mental illness-we are restigmatizing those facing mental illness as crazy murders. Certainly, there are people who will choose to believe that in stating this I mean that all people who are mentally ill are potential murderers, but that could not be further from the truth.
To put this in perspective, let's take three people...we'll call them Abe, Bob and Carl. Abe, Bob and Carl are all physically ill. Abe has a norovirus, and is puking and crapping his brains out even as we speak. Bob fell down skiiing and tore muscles in his back and Carl has a terminal form of cancer and less than a month to live. NO ONE in the real world would say that since Carl has a really extreme illness, Bob and Abe aren't sick, and don't need treatment. No one would say that since Bob and Abe don't have cancer, they aren't really sick, and no one would say that since Bob did it to himself he's not sick. We all agree there is something wrong with their bodies.
When you murder, you can murder because you can't think in an ordered manner, you can kill without understanding you're killing. This is the kind of murder where we find people not guilty because of their mental illness. You killed, but you could not understand your own actions and you can't understand a trial or a sentence. Your mental illness, in this case, is so big that it overrules everything else. You're the mental illness version of the guy with cancer, your brain is a runaway train and it may or may not be able to be stopped. We do not judge if the damage to the brain is caused by a genetic condition, by a trauma to the brain, or your drug use, or if your mom stabbed you in the brain with a carving knife...we recognize that whatever the cause, you have a severe impairment as a result of it.
You might murder because of a mental illness that is transient. You may have the mental equivalent of poor Abe, above. Most people will get a norovirus at least once in their life and will describe it as the sickest they have ever been. I, personally, have had puke-inducing chemo pills that I would rather take than ever face a norovirus again. You could be so sick you didn't understand what you were doing at the time. You could be sick from a virus, an injury, even from a trauma caused by your own emotional reaction to stuff. You could be sick from an underlying condition that only bothers you sometime (consider the kids who get confusional migraine, or the people who kill themselves during cluster headaches.) You may or may not have known better when you murdered, that's a question for the courts, but you certainly were not capable of ordered thought when you murdered.
You might murder because of a community or shared delusion. There are people who want to call these cultural reasons...reasons of poverty, gang violence, etc. If you have been raised to believe the only way to escape your situation is murder, you have believed the delusion of another...maybe that delusion was plausible, but it is still a delusion. If you have been taught by your mother that your only hope is joining a gang where you have to kill a person to become a 'man,' or you have been taught by your mother that your only hope is killing a person with green eyes because they are lizardmen from planet X and they will eat you, you have believed a delusion, and we treat them (medically) the same, with education (and sometimes medication.) Think about not only people 'raised with' these shared delusions, but those who kill because of a powerful leader, the people we often call (incorrectly) cult leaders. These delusions might make perfect sense at the time...your brain may, in fact, be considered 'normal,' but you're still under the effects of a mental illness...even if the organic damage behind it (if any) resides in another person's skull.
Any of these reasons is a mental illness. They do not represent the full spectrum of mental illness in the universe. THEY DO NOT REPRESENT MOST MENTAL ILLNESSES. The vast majority of mental illnesses are nothing like this. To go back to the 'physical' illness example, most of us have vomited at some point in our lives....but most of us have not vomited up our organs. Mental illness is like puking, we're all going to face it at some point, either as a result of our own actions (ie: drinking too much), as a result of stuff we can't control (a blow to the stomach, food poisoning), or just dumb genetics...but the existence of these mental illnesses (I don't want to call them minor mental illnesses, because they are serious to those experiencing them) and the fact that they are mental illnesses doesn't change the fact that there are more severe and life-threatening forms of mental illness.
The events in Newtown do not call for an increase in sane gun laws or better access to mental health services. Either of those things could've helped. The events call for BOTH sane gun laws AND better access to mental health services...because no one who is capable of thinking in an ordered, rational manner is going to murder someone (They could kill them accidentally, or as part of a job, and that's a valid discussion, but it's not murder.)
If anyone, on either side of the debate brings up that OR... If they say that we can have (or that we need) sane gun laws or better access to mental health service, shout them down. We need BOTH.