Towards the end, as Chris is attempting to leave the house, he’s jumped by somebody. It’s not the brother he pool-balled in the basement, is it? And as they’re struggling, he reaches for the doorknob. What is it he does with the door that defeats his aggressor?
It’s the brother. Chris knocked him out earlier (causing a bleeding head wound) but did not kill him.
Chris waited until the brother kicked the door closed again, then when the leg was pushing against the door, Chris stabbed him with the letter opener get got from (and used on) Mom.
Yes, Chris realized that he was going to put his leg up for the door again, so he reached for the knob, and then stabbed him in the leg.