Jack Levin, a university criminologist, writes on CNN’s In America blog that most Americans are made uneasy by mass killers who turn out to suffer from a mental illness. Levin offers a simple explanation — anyone may fall mentally ill.
He writes,

Part of the reason may be that the attribution of illness sounds like an excuse rather than an explanation.
The label “evil” distances the killer from “normal” people in the minds of most Americans, but illness does not. Millions of Americans suffer from depression; millions more have anxiety disorders, and they wouldn’t kill anyone.
Most individuals do not regard themselves as evil; they have a conscience, feel empathy for victims, and are remorseful when they do the wrong thing.

Instead, of being uneasy, Americans should destigmatize mental illness and work to assure that treatment is available for the mental ill before they become dangers to themselves and others.
Read more here: Opinion: Mental illness an explanation, not an excuse for Colorado shootings – In America – CNN.com Blogs.