"Kagemusha" by Akira Kurosawa. In English, "Kagemusha" translates as "Shadow Warrior".

http://www.imdb.com/title/tt0080979/?ref_=sr_1

“Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you.”