Ja no Michi

The manga centers on two friends Momo and Tatsu. When Momo lost his way and encountered dangerous snakes in the mountain as a kid, it was Tatsu who rescued him. After the incident, Tatsu becomes obsessed with the idea of eternal life, and he believes that killing a large number of snakes will give him immortality. Despite being treated as a "weirdo" by everyone, Momo keeps on looking after Tatsu. But after one wrong word said, the two friends get into a fierce argument and get into a surprising fight, leaving their families and normal everyday lives behind.

(Source: Anime News Network, edited)