Board Thread:Questions and Answers/@comment-26242189-20160422234238/@comment-27701762-20160423014429

Presumably because the attacks from the various monsters are directly hitting the soul. Whereas in normal circumstances, it is when a human's body dies that the soul persists.

I would guess that Asgore (and any other monsters that don't want to kill you but know that you're human) doesn't know his own strength, so that he kills Frisk by accident.