Awful mum. Shame on her for allowing her son to claim credit for her depression (and allowing so much anger to build up in her son). The fact that both kids were overweight also is a red flag for me. Her "I have no recollection of that" denials were chilling. And this was her in front of the camera.
I have to say that I felt creeped out every time the child was speaking - it felt disturbingly insincere - I felt VERY manipulated. The way he glanced at the camera while talking to her didn't feel quite right, like he was playing to an audience. This could just be a self conscious response to having a camera in the room, maybe.
His aggression was also far out of proportion to the actual events taking place, and he seemed to be very easily slighted by seemingly minor comments. I felt a chill when he minimised by saying "I lightly slapped her". When they were talking with each-other, it seemed like a battle of wills, and that his will was much stronger than hers. I'm guessing that she was relied on physical power and manipulation when the boy was younger, and that those were no longer working now that he was smart enough to understand. I think he might have been spoiled or neglected in some way, and never shown good boundaries... I got a confusing sense that this child was either:
a) very insightful and would eventually healthily navigate his way out of this situation; or
b) manipulative, lacking empathy, very sensitive to his own feelings, and basically a monster in the making.
I feel like lots of sympathy for him in either case, I just hope he doesn't turn his substantial anger towards other innocent people in future. In some ways I see some similarities between this and my relationship with my mother - although in my case my mother was not really abusive, but rather made some irresponsible big decisions. I built up hostility after some abusive experiences at a new school starting at age 10, and I became depressed and passive aggressive at home; She didn't respond with calm assertiveness, but rather would get hurt or angry or sad. Once in a while, we would have a fight with lots of interruption, raising voices and eventually one of us would storm off to our room. When it was me, I would just be in my room angry or crying, then she would come in and ask me with genuine sympathy what was wrong, and I would break down crying and we'd talk for a bit. It seemed like those were the only times after the age of 10 when I felt truly listened to and that I could release my sadness and anger about feeling worthless and rejected at school. Around the 8th time this happened, I realised that I could use this 'crying and getting sympathy' in a manipulative way, and the thought repulsed me so much that I vowed never to allow that to happen. It was always sincere. Anyway, I'm in therapy now trying to figure out how to deal with my relationship with my mum. We still often have aggressive, subtly dominant conversations filled with interruptions and fast paced monologues, now through intellectual topics, and I can still feel myself being passive aggressive with her.