I just watched this and the mum seems so devoid of actual emotion. I’m not a body language expert, the way she shakes her head is like she’s trying to convince herself he didn’t deliberately stab Scarlett. I read the article shared further up thread (the dreaded DM) and a lot of the comments are mentioning DV, abuse, not a normal family situation with the late dinner.
I know we will never know and Scarlett doesn’t have a voice but I think we can all tell the mum is protecting him, perhaps she is terrified of him or he’s brainwashed her into believing what happened didn’t actually happen. A comment mentioned a jokey situation with grapes and Scarlett telling him not to be a wimp shortly before what happened. I wouldn’t be surprised if he was violent and with the drugs in his system just saw red.