Whose testimony prevails if a killer claims a person slain in the Abode of Islam was a disbeliever, and the guardian denies it?

Primary text

If a person kills someone in the Abode of Islam and claims the victim was a disbeliever, while the guardian denies this, the statement of the guardian prevails. This is because the default status of the region is Islam, which is why the status of Islam is ruled upon for an abandoned infant found therein.