When Jon and Karl left the bedroom, Zoe (Jon's 14-year old Goth sister) snuck in and grabbed the device off of her brother's bed. She had been spying on Jon and his friend when they were using the device. She saw what it did and was immensely interested in using it. And why not? Zoe always dreamed about other people being like her. And now she had the means to make it happen.
She walked downstairs and saw Jon and Karl playing one of those first-person shooter games on the TV. She pointed the device and pulled the trigger.