His first job was at a small computer shop where he wrote business software. He felt the job was boring and with some friends started developing PC games in secret, on work machines and while lifting wages. This is how id software was born. Due to that book, I think he will have a hard time convincing a jury he is innocent.
edit: its even on wikipedia
Softdisk, [...] hired Carmack to work on Softdisk G-S [...] introducing him to John Romero and other future key members of id Software such as Adrian Carmack [...]. In 1990, while still at Softdisk, Carmack, Romero, and others created the first of the Commander Keen games, [...]