It's a common assumption that the soul is immortal.

It is what people tell themselves when loved ones "move on" or they themselves are about to die, to make the finality of death less daunting. Death is, of course, just the "next great adventure".

Even the being known as Lord Voldemort believed in this - hence his believe that the horcruxes, which would anchor his soul to he realm of the living, were the ultimate way to ensure his immortality.

Too bad for him, really.

In truth, after death the soul only lived on as long as there were people thinking fondly of the deceased – and truthfully even his followers hated Voldemort. Strike one.

And horcruxes..

Well horcruxes did anchor the soul to the world of the living, but splitting the soul also split the amount of "live" left in the soul. Strike two.

By the time Lord Voldemort went to Godric's Hollow to kill the Potters, most of his horcruxes – which he had created as long as 40 years ago – had already broken down and his own time had already just about run out.

The killing curse is a very curious thing, too. All people knew about it was that it killed.

What they didn't know was that when the curse had been invented – interestingly enough by the man who would later curse some fellow wizards with a curse that would turn them into Dementors – it had been called the "soul killer".

It put an enormous amount of pressure onto the soul, aging it far quicker than it should – killing the victim even if they still had live in their body.

Lord Voldemort was hit by his own killing curse that night and it affected not only the piece of his soul left in his body, but the entirety of his soul. Strike three.

The amount of strain on his soul was too much. That night Lord Voldemort well and truly ceased to exist. He was out.

Thus is the folly of playing around with things way beyond your own understanding.