As Kent mentioned before, the factor a defendant claims in mitigation doesn't need to be a legal excuse.  And that is mentioned bluntly in this article from the journal Nature which discusses the nascent use of psychopathy as a mitigating factor based on brain scans:

The purpose of the work, Kiehl says, is to eliminate the stigma against psychopaths and find them treatments so they can stop committing crimes. But Dugan's lawyers saw another purpose. During sentencing for capital crimes, the defence may present just about anything as a mitigating factor, from accounts of the defendant being abused as a child to evidence of extreme emotional disturbance.