I think that she fears what learning that he’s the result of rape could do to him
This is a good point - it would be a terrible thing to find out.
I’m not that convinced by Faisal murdering Joanne. He doesn’t seem the type to get violent.
Disagree here - there is a phrase "Beware the wrath of the quiet man".
People who bottle things up eventually reach a stage where they explode with emotion, acting far more violently than they would if they had had the chance to get the weight off their chests sooner.
Pfizer (nice nickname, whoever thought of it) has been a simmering volcano of frustration, rage and fear for a long time. His wife doesn't listen to a word he says, his daughters love him but regard him asa joke, he is short of cash, the unflavoured son-in-law, a dogsbody in his FILs business, frightened the police will find out he's been supplying drugs and terrified that his daughters are going to be abducted, raped and murdered by the gangsters. He can't see any way out of his dilemma - poor, desperate Jo was just the one in the firing line when it all came to a head.
I have a feeling though, that if she hadn't laughed when he said "I'm stronger than I look", it might not have got to that stage. He obviously has low self-esteem and has probably been the butt of family jokes for years, and is also aware that he is unable to protect his family, and that laugh hurt.