passivechara:

I think the shattering is a visual indication of death.
But like we see in the True End fight a human soul can mend itself. But not all humans have the level of determination Frisk has. So when a normal human (or a human who willingly refuses to use their save file) gets killed by a monster it probably looks like this:

But whenever we get a game over in Undertale it’s more like:

((and in the final battle Frisk’s Soul is so determined that it just outright refuses to die! Hardcore kid right here))