The FBI has released the contents of Brian Laundrie's notebook that they found next to his body, and it includes his confession of killing Gabby Petito. According to the notebook, Brian decided to end Gabby's life after she supposedly injured herself after a fall. And that he "thought it was merciful, that it is what she wanted."
(See the notebook confession below)
Brian wrote in the notebook:
I ended her life. I thought it was merciful, that it is what she wanted, but I see now all the mistakes I made. I panicked. I was in shock. From the moment I decided, took away her pain, I knew I couldn't go on without her.