Pa didn't beat Teddy because he killed her, he beat him because he probably had sex with her after he killed her since Woody probably told Pa that he saw Teddy ripping her bra off. We know that Teddy is well... kinda horny since he was also the father of Fanny's baby... or atleast I'm guessing so because he tells Fanny that Pa would "whoop him bloody like last time" when she walks up to him with lust in her eye after spying on Jeff. And we all know that Pa is totally against sex before marriage because of the speech he gave: "There will be no devil's play in this house!"
The flashbacks were intended to show you where Cynthia was coming from... why she was in the hospital at the beginning. She sees Fanny giving the baby a bath and it pushes her over the edge because she's seeing it as HER baby.
I'm guessing that Cynthia joined the family because she was mentally unstable to begin with and just went crazy from everything that was going on...
I know the baby scenes were meant to show us why cynthia was mentally unstable I meant what was the point? why did they make her mental - it had nothing to do with the story. And I still don't get why she joined the family in the end!
I just dont see whats so hard to understand about her joining the family. She was IN SANE! INSANE! IN........SANE. Geez, she was koo-koo before hand to be pushed over the edge at the end with everything happening and her flashbacks. She went over board and killed THEM too! Again, she was insane. Its an 80s horror movie, dont spend so much time deconstructing the sub-context. Or, ah, even looking for it.
Yeah, I mean watching it as a parent these days I absolutely CRINGE and panick a bit when they run through her past and also seeing that mumified baby. That's just WATCHING it. If it actually happened to me, i'd be mental. She joined the family, but she also killed them afterwards.
Either way, it's too bad she became homicidal and then retreated into a psychotic fantasy in the end like that. Usually in movies like this the final girl would have escaped from the house (preferably setting it on fire first).