Why pass away means die?

Answer 1

'Pass away' is one of the oldest euphemisms known in English. It was coined in the 15th century in Britain, when most people would have believed that the departing of the soul of a dead person was a literal physical event. Indeed, 'passing away' didn't mean dying as it does now. When wakes were held for recently deceased people the attendees believed that the dead person could hear and comprehend everything that was being said: it was only later, when the funeral rites were complete, that the dead person 'passed away' and began the journey toward either Heaven or Hell.

As such, 'pass away' wouldn't have been considered euphemistic but merely a literal description of events.