the best they could come up with was his Dad didn't support him at football?
The incel part was mentioned out of the blue and hardly referred to again.
This is kind of the whole point. Behaviour plenty of people see as "normal" is actually incredibly damaging and it is perpetuated through the generations.
That was not a healthy family dynamic by any stretch. The fact that Jamie wasn't being physically abused like his dad was, or starved, or entirely ignored by his parents doesn't mean he was getting a good upbringing even is his parents were doing their best.
When Eddie was kicking off that they all had to go to the shop, Amanda looked genuinely scared. The feeling of walking on eggshells so as not to exacerbate his temper really was palpable and it was clear this wasn't the first time from the way she and the daughter reacted to it. This is the man that tore down a shed when he was angry. He chased and manhandled a teen, and then trashed his own van before screaming at a security guard. That isn't normal, or certainly shouldn't be, and living in a household with that kind of explosive anger is damaging. Yes, it was a high stress situation, but that is clearly how he handles stress, and violence is what he has taught his son.
Even little things like the fact Eddie pointed out that his father hadn't written his birthday card and it was (presumably) his father's wife. More gendered stereotypes being enforced through the generations. It might be reality, but why is it? Why isn't a father writing a card to his son rather than getting his wife to do it? Jamie's experience of men is that they have explosive anger and are violent (whether to other people or objects) and leave the "girly" stuff like cooking and sending cards to the women, who are expected to do it and listen to what the man says (e.g. I don't care you're halfway through making breakfast, stop immediately and come to the shop with me).
The way Eddie went completely silent when Jamie said he was going to change his plea was a direct reflection of him turning away at football. He's embarrassed/disappointed in his son and therefore can't cope with interacting with him, so he ignores him. That silence went well beyond just trying to process a few emotions and figure out what to say, and then mum comes along with inane chatter about food (of course, she's a women, therefore has cooking on the brain), trying to distract everyone and bring tensions down, but all it does is dismiss the genuinely important thing Jamie had just communicated about his plea. Jamie has no respect for her, just goes straight to his room and when she tells him to turn off the light (in what sounds like a pretty weak and ineffective way) he does that, doesn't even say goodnight, and she doesn't even bother checking to see whether he's actually gone to bed. It's all completely dysfunctional, even though the parents clearly love their son.
The way Jamie was standing over Briony to intimidate her, and the way he was speaking to her - that smacked of family influence, not just online. "Get that through your fucking little head" is exactly the kind of language a man with explosive anger and misogynistic tendencies would use towards his wife that disagreed with him, as is using physicality to intimidate, and it wasn't a stretch to think "I bet he learnt that one from his dad". Women being lesser to men is some this his parents and grandparents have been reinforcing his whole life through their actions, so the jump to "and therefore as a man I am entitled to women" isn't exactly hard. It's a dynamic that absolutely primes boys and men for the manosphere because they feel like they are more deserving because they're men, it's the women's fault for not falling in line, and it must be the 80/20 rule causing it. It's their way of rationalising why they aren't getting what they want and shifting the blame away from themselves or acknowledging that women/girls have agency and other priorities.
With that expectation of power over and obedience from women he gets from his home dynamic, paired with the red pill position of "women want chads and are sluts only good for sex", plus hormones and being 13 with all that comes with it...I don't think it's that far fetched that he'd stab a girl that was laughing at him publicly online, mocking him, and in his view didn't want him even when she was at her lowest because he's a beta and just wanted chad Fidget instead, which just shows how stupid she is because Fidget is clearly an idiot (for showing everyone the pictures before getting more).
The Atwood quote is incredibly apt. Men are scared women will laugh at them, women are scared men will kill them.