Jim can't cook. Not just "it doesn't taste good," but "it's not actually edible." He and his roommates (Ian, Elias, Jacob, Noel, Olivia, and Cassidy) trade off days for making dinner. Ian is the best cook, and even with Ian's assistance, the days that Jim makes dinner usually end in disaster. Jim usually just gets food delivered on these days.
Because Jim got horrendously sick the first few times he made himself food with meat in it, he is now a vegetarian.
When he was fifteen, Jim's mother walked out. This left Jim to bear the brunt of his father's abusive tendencies--from taking out his emotions physically to mental and emotional abuse, Jim bore the brunt of his father's anger in order to protect his brothers, who were only nine and seven at the time. Combined with the bullying at school and the encouragement from Harry, his father's abuse is what drove him toward killing Carl Powers.
There is one thing that really terrifies Jim: thunderstorms. He knows it's childish, but they remind him of his father screaming at him for his whole life. If he hears thunder, it's extremely difficult for Jim to do anything but freeze up.
He loves horror films. They give him ideas for the fantasies he has of killing people who annoy him.
While he is asexual, Jim is aware that he isn't a bad-looking man and is perfectly okay with using sex to get him what he wants.
He actually does like children's television. He enjoys turning it on as background noise when he needs to turn off his mind for a while.
During the month of December, Jim picks twenty-four children at random and tells them the truth about Santa Claus. It's his own private mode of entertainment during the absurdity of the Christmas season. (The Moriarty family never celebrated holidays in the conventional sense--if the boys were lucky, they would be awarded with half an hour of extra play time and extra dinner.)