incarcerated homicide offenders had reduced gray matter volumes in the medial and lateral temporal lobes, including the hippocampus and posterior insula
...and indeed I've wondered why we don't test for conditions in the incarcerated population that have known potential for altering behavior, eg Toxoplasma gondii see http://wwwnc.cdc.gov/eid/article/9/11/03-0143_article for example.
Assuming for the sake of argument that we could soon reliably identify symptoms of a homocidal disposition, what does this mean for criminal justice and law enforcement? From a purely utilitarian point of view it makes sense to monitor or even confine such people, but from a libertarian (philosophical) point of view, we could not subject them to penal confinement as people can hardly be held criminally liable for deficiencies in their own brain structure, which might be the result of genetics, malnutrition, or childhood trauma.
Some sort of medical incarceration is the obvious alternative, but this is legally problematic. A Supreme Court case O'Connor v. Donaldson (1975) (among others, but this is the big one) made it very difficult to commit someone for confinement in a mental institution without clear evidence that they present a danger to society. Arguably, the US went putting people in mental institutions too casually and without regards for their rights, to a position of not providing sufficient mental health services for all but the most unhinged patients. While this technological advance might one day provide a reliable gauge of a person's homicidal capability, that's a long way from identifying how or when such behavior might manifest.
At a less extreme case than confining someone, should we allow someone with a measurable homicidal tendency to purchase guns? If we forbid them doing so, how do we avoid a tiered approach to citizenship, in which the rights of some are abrogated pre-emptively through no fault of their own actions, but a 'fault in their stars'?
From a different angle, this is also an interesting result for attorneys involved in death penalty defensive work - if MRI scans, which the courts and public are highly familiar with, can reliably predict homicidal characteristics, then the question of criminal responsibility becomes much more difficult to answer, since people have no control over the structures of their own brains.