How come he can't see the therstrals after killing Quirel?

The movie made two major changes in regard to Harry vs Quriel.

1: The protection spell only takes effect when Harry touches Quirel in the movie. In the book, it happens when Quirel touches Harry, and only after Voldemort attaches himself to Quirel.

2: The spell turns whatever body part Harry touches to stone, which then crumbles to dust. In the book, it only causes painful burns and blisters. Quirel runs away in pain after Harry passes out, and Voldemort decides to drain his life and escape since Dumbledore and the teachers were coming.

These two changes make it so that Harry, at 11 years old, kills a man in the movie. This change is what makes the plot hole of the Thestrals.