The organization or individual that had trained Boo Radley must have had a very clear aim in mind.
They would first have to find a suitable candidate, one with a natural tendency towards silence and preferably no family. Then, whatever personality existed would have to be destroyed and rebuilt in an amorphous design. The man that result of this would have no ego of his own, no defining trait, nothing to sway him from completing the mission.
Then you'd take the man with the clay mind and mould him. Teach him to move silently, or in concert with every breath of wind or creaking branch. Show him how to use every shadow available, every turned back or blink of the eye.
Demonstrate to him how hands and feet can cripple and paralyze, and then later how force of will can raze and devastate.
From the blank slate that is his ego, enable him to adopt new ones and to copy those of others. Educate him in the crude skills of physical mimic, but also how to imitate every mannerism and characteristic.
The finished product would have been a superb spy, capable of avoiding or fooling any security measure. He would also be impossible to recapture, should he prefer to take his own path in life.