It's the Law and Order theory of murder: Every crime speaks to a recognizable social pathology. A sexually frustrated misfit declares his vow to seek revenge on the women who refused him and we call it misogyny. A white supremacist opens fire in a black church and we call it racism. A Christianist opens fire in a Planned Parenthood and we call it religious extremism. In all cases the killers, whatever their grasp on mental health, have declared their motives and we accept them at face value.
We have a habit, however, of seeking such explanations everywhere. Columbine, we told ourselves, was about bullying. The crazy news guy, a black gay man, kills his former colleagues on camera and we consider it indicative of our culture's continuing dehumanization via the media. A college dropout opens fire at a congresswoman's meeting and we call him a Palin-inspired right-wing nutcase. The first claim is inaccurate. (One killer was a diagnosed psychopath who would have killed someone at some time and was just waiting for the opportunity. The other was a troubled kid who fell into his orbit.) The second claim might be true, but seems a little tenuous. The third claim was inaccurate as well. The killer had recently suffered a psychotic break.
Then there are the murders that defy anybody's explanation, like the shut-in who killed a classroom of kindergarteners.
There are two reasons, I believe, for the desire to define motives:
The first is that we all seek out examples of the extreme results of social pathologies we fear most. If you are a woman who has been harassed at a comic-book convention, the story of a geek who kills because he can't have sex will strike a chord. If you are a neoconservative who believes we should be bombing Syria, the story of jihadists at home will strike a similar chord as well.
The second is that we need explanations for people who do evil thing we could never see ourselves doing, or to see the roots of these killings as related to social problems, problems that we might, in the long long long run, be able to solve.
This is not to say that the motives don't matter. I believe they do. I also believe that the motives in all cases are far more complicated and far more individualized than Facebook memes and political posturing suggests. I admired the Rolling Stone cover story on the Boston Marathon bomber, because it understood that as much as we would like it to be the case, our most evil citizens and our social pathologies cannot be summed up in a paragraph.