the murder's relevance

The murder doesn't have any direct relevance to the end of the world. It served as a mechanism to connect David with Chris (who'd he'd been dreaming about) and Charlie.

The murder occurs because Billy discovers (or is exposed to) tribal law secrets he shouldn't have seen or known about. As Chris says at one stage ..."he hadn't been through the law." (i.e. he hadn't been initiated into or given permission to know about those secret areas).