"He who fights with monsters should be careful lest he thereby become a monster. And if thou gaze long enough into an abyss, the abyss will also gaze into thee." Nietzsche - Beyond good and evil.
This quote symbolizes the neverending struggle Guts is facing. By trying to eradicate his demons, he's slowly becoming one himself. This poster is about Berserk by Kentaro Miura, more precisely on Guts. "Gaze into the abyss" illustrate the omnipresent temptation the protagonist fights against.