PTSD
Poor Ellen.
I doubt she knows exactly what happened to her rabbit, put in the same day you see that she's in shock once she's in bed after that incident.
Then shortly after her parents are having a fight that wakes her up and watches it happen.
At the end of the movie with the battle of screaming and a gun shot who knows what was going on for her.
There are so many kids nowadays that have Post Traumatic Stress Disorder, and even if the same percentage of people are psychos that add to cause stress on, the population is multiplying and so is all of what's wrong.