This question already has an answer here:
If Harry had witnessed his mother's death at age one, how come he couldn't see the thestrals on the carriage?
Many people may argue that he hadn't registered that she died at the time. And JK has stated the same. But in The Prisoner of Azkaban, Harry had witnessed it again when he went close to a dementor. How come he didn't see them in The Goblet of Fire?
Some people see a loophole in this though. In Sarah Buckley's theory which I will put HERE, It could have been Voldemort's soul in Harry that heard the voices.
In The Prisoner of Azkaban, Lupin that dementors affect him so much because.
"It has nothing to do with weakness," said Professor Lupin sharply, as though he had read Harry's mind. "The dementors affect you worse than the others because there are horrors in your past that the others don't have."
But, as I have stated before, it is confirmed that Harry cannot fully remember his mother's death. Sarah Buckley theorizes that the horror was when Voldemort was defeated. Her theory was,
"It wasn't Harry’s soul who reacted so badly to the dementors, but the piece of Voldemort’s. The night Lily Potter died and Voldemort was defeated was the worst experience Voldemort had ever had. Consider also that the memory stops (and Harry faints) just a few seconds after Lily screamed and died, which would place the end of the memory right around the time Voldemort tried to Avada Kedavra Harry and was subsequently defeated - killed, or as close to as possible."
But I also disagree with this theory. Why? Well, in The Chamber of Secrets Dumbledore says
"You can speak Parseltongue, Harry," said Dumbledore calmly, "because Lord Voldemort--who is the last remaining descendant of Salazar Slytherin--can speak Parsletongue."
And that is because there is a fragment of soul from Voldemort in Harry. This is my evidence that traits can be passed from Riddle to Harry.
This is also evidence for the dementor-Harry theory. But since harry can't see thesterals until Order of the Phoenix, not all traits can be passed down.
And in The first book he killed Quirrel! Shouldn't that count as witnessing someone die! I mean- There are so many plot holes with this subject I don't think I can solve the answer of this question. These are all the plot holes I can find, but I what to see an other opinion.