Erich had been dispatched on a mission to silently eliminate any threat behind the scenes to the war effort. Whether within the boundaries of the Central Powers, and the borders of the nations that it was composed of. Or abroad. Deep behind enemy lines, he was the type of man best left to his own devices after he had been given a task to complete.
Frankly speaking, the less Bruno knew of how the man completed his task, the better it was for everyone. An apt comparison would be to say that Erich was like a fire and forget missile system. Simply point him at a target, lock him in, and pull the trigger.
After that, one did not need to concern themselves with the matter any longer and could engage other targets of priority. There was nobody better at the job Bruno had tasked his old friend with than the man himself.
