Zaniku no Natsu

Shizumaru, melancholy and lost, wanders through the land in search of the demon he has no memory of. Panicked but skilled with his techniques in battle, he unintentionally kills many of his opponents. Not knowing what to do, Shizumaru travels again and repeats this ugly cycle with every new encounter. He is warned to stop his activities by Hanzo, lest the boy become a demon himself.
(Source: SNK Wikia).