It’s scientific fact. Read the book referenced in the Forbes article. When a person with brain damage can’t convert short term memory to long term memory but can form new habits, what’s going on? The what’s going on is a recent discovery.
And even if the guy had a serious brain malfunction impelling him to kill at the first sight of flung popcorn, he is still guilty of manslaughter. He still was packing when he shouldn't have been. He still, by your own post, expressed horror at his own choices after he killed a man. And he deserves to be punished for not having developed self-control, especially after having sworn to "protect and serve" all those years. Face it. Even if he had run over a man with a car "by accident" because he's old, he would face jail time. And would deserve it.