The Space Marine is actualy a good analogy, as there have been case of Space Marine being killed by traitor.
They dont need to be upfront to kill him, they can just help the enemy taking him down, using a grenade when he sleep, straight out poisoning him (it's easy to find things to poison someone on a frontline).
Sure it would be dangerous, but in this kind of war you know that if you follow such a dude you will die, you will be thrown away like a trash.
When this happen many men dont care about the possible failure, they take their chance because in the end if they succeed, they have a chance to see the end of the war.