No way would Tyrion kill his own nephew so publically. I’m sceptical about him killing his own nephew at all, but if he did, he’d be much cleverer about it than doing it when he was the one handing a cup of poison to the king in front of a crowd of scpectators. It wouldn’t even work as a double-bluff.
Cersei, no. She loves her children. And she could have killed him any time. She’s probably the only one who really could have killed him any time, actually - she has the access and the contacts and she even has the poison handy from when she thought she’d have to give it to Tommen (and take it herself) in the siege. If she’d gone out of character and decided to murder her own son (giving Tommen the poison in the siege is not the same thing) she would have done it in a different way.
Olenna, well, she wasn’t the one who set up the marriage, was she? Margery and Loris seem to have set that in motion all on their own. Then she came to King’s Landing and saw what a monster Joffrey was; she might have been sceptical Margery would even survive the wedding night, or she might even have wondered if Margery would end up as bad as Joffrey.
Margery did take delight in shooting the bow, after all; she is power-hungry; she does try to feed the poor, but that could just be propoganda, or it could show that she does have principles of a sort, which actually makes her more dangerous.
Margery, no. She was really happy about the wedding. She wanted to be the Queen. I think she wanted to get pregnant first, at least.
Anyway… Other parts of the episode: poor little Junior Stannis. Her mother genuinely thinks it would better for her to die and “join the Lord of Light.” Her mother really is that religious. And all her other children have died, so why not this one too?
Stannis did show some humanity in straight-up saying no to his daughter being harmed. That could be his breaking point - Melisandre and his wife want to kill his daughter, and he tells them to fuck off. All hell breaks loose. Or possibly his wife tries to do it herself and is saved by Ser Davos and maybe even Melissandre.
We haven’t yet seen anyone in Westeros kill their own children, have we? Everything else, yes, but they protect their own children to the death.
Charlie, it’s understandable that you mixed up Stannis’s wife with Lyssa Tully. They look alike, and share the same expression of fevered mania. OTOH, if there’s one person Lyssa won’t kill, it’s her own son. Drive him insane, yes, but not kill.
The Bran bits were boring and he seems to have some sort of rapid aging disease. The actor’s always been good before, though, so there’s hope.