For a start they're not in the Army. The armed forces come under the MOD, MI6 comes under the foreign office and MI5 comes under the home office. You won't find them on patrol in Hellmand like army infantry, infact you wont find them anywhere, they're too good for that. They gather intelligence and probably but certainly not confirmed are involved in direct action often alongside SAS, SBS & SRR. Read a bit about them and see if you think they get involved....
The marine broke the geneva convention and got caught, there isn't any defending that. We can't just say "f**k it the convention