I can't believe that he definitely knew that he was possibly involved in attacks against the West.
These training camps have been in existence for many years, mostly well before 'Al-Qaeda' even eventuated. Trainees from ALL over the world trained in these, and not necessarily for the same purposes. A small percentage of these were approached for attacks against civilians etc, and they were free to (and they did) refuse.
I have no doubt that he was a mercenary, but I'm not convinced that the newspapers in Afghanistan camps were of the standard that we enjoy in our own Mess.
Besides, he did at least TRY to join the Australian Army, didn't he?