The judge’s recent decision allowing the State to introduce evidence about Justin Ross Harris’s marital problems in 2014—at the time he left his 22-month old son, Cooper, to die in a hot car—is a boost to the prosecution and a setback for Harris. In a hearing last month, the defense had argued against the admission of this evidence and more. The judge is allowing the prosecution to introduce evidence of marital infidelity, sexting messages he sent to underage girls, computer searches about weather and the effects of heat on the body, and Harris’s research in the law. This circumstantial evidence points to a motive for murder—that Harris wanted to live a life free of the responsibilities of parenting and that he wanted out of his marriage. Motive is not an element of any of the crimes charged. Except in the…