Why couldn't Harry see the Thestrals after he killed Quirrell?

Hello guys its me and welcome back to another episode of answering questions no one asked because I'm out of ideas at the moment.

There have been many instances where Harry has witnessed someone—to quote the great Ronald Weasley—"snuff it" before the 4th book. For example, in the Philosopher's Stone, Harry literally sees Quirrell die, and the in Goblet of Fire, he can't see the Thestrals at the end of the book! Some people say its because he didn't have time to register Cedric's death, and okay, but surely he was able to register Professor Quirrell's death in 3 years? 

My answer to this is that maybe Harry couldn't see the Thestrals because he didn't actually see life being wiped out from Quirrell's body as he fainted before witnessing that. He only saw Quirrell's skin blistering, but he didn't actually see him die. 

This post is particularly for the people who've only watched the movies and think that Quirrell turned to stone and crumpled. N O  H E  D I D  N O T.