Imagine Spider having the gift to commune with the spirits of the dead, Na'vi, and humans. Spirits are everywhere in the forest (more appeared after the great battle) — watching him practice his archery, guiding him back to Hell's Gate in the dark, and whispering stories to him. Some are peaceful and content, while others are restless, especially those who died violently or don't know they're even dead.
Naturally, Spider keeps this to himself because he's afraid of being further ostracized. Instead, he decides to see this gift as a good thing because at least he has friends to talk to. He doesn't care if Lo'ak thinks he's weird for talking to himself or staring off into space. Spider sees fallen Na’vi warriors from the past, human researchers and soldiers who died on Pandora, and even little children who perished when the Tree of Souls was destroyed. They sometimes speak to him, asking for his company or to listen to their stories.
Maybe it's not healthy to spend hours alone with just the dead for company, maybe it's not normal that he prefers sleeping outside in the company of spirits then with the living back at Hell's Gate, and maybe he made a mistake venturing off alone to live in isolation because he's found more belonging with the dead then with the living.