If you are outraged by Marcus' theory you probably should read a full version. The theory was ploted by a Michel Brun. He wrote a book about the incident. It involves tens planes from both sides engaged in air figths. Some of them (not only KAL 007) were presumably downed. Marcus at least occam-razored that staff.
While many people in Russia believe that the flight throuh Soviet air space was intentional, the Brun's theory isn't very popular.