I f'in LOVED THIS MOVIE!!! After that first scene that you mentioned, I looked at my SO and said, "That is the most vile thing I have ever seen!" and then we high-fived!!
I agree about the sound effects, too. Especially the closet scene.
The ending is, IMO, pretty weird and lame, but it is of no consequence whatsoever. By the time you're done watching it, you're so bowled over by the actual movie that the ending is just a sidenote, anyway. "High Tension" really lives up to it's name. I was on the edge of my seat the entire movie.
Best. Horror. Movie. Ever.