Guts' seemingly endless struggles against the Apostles is a nice illustration of one of Nietzsche's most famous quotes: "He who fights with monsters should look to it that he himself does not become a monster. And if you gaze long into the Abyss, the Abyss also gazes into you."

All those battles against hellish abominations made him walk the edge and made the bloodthirsty Beast of Darkness inside him grow more and more powerful to the point he almost succumbed to it, as manifested by the Berserker Armor's helmet's appareance changing to look like the beast within when he unleashed it. But thanks to Schierke and her magic, he did not turn into a full-blown monster.