Explain (nearly) everything. Mental illness is so frightening to others because it's irrational. The ending would have been more consistent with the reality of a psychic break if she had left out Junior's preposterous "Night mom. Night son" monologue. Skip that and go directly to Day Three and and out-of-the-blue murder, and you have something realistically shocking. You can't explain when someone snaps, so don't explain it! It had the feel to me as I watched it of an unskilled author's felt need to explain something so she makes the explanation too plain--and puts it in the mouth of a character who has hardly ever fucking spoken! That is lame.