...the mom and Madeline both rise from the dead to kill? And why did Roderick poison Madeline in the end?
shareThe mom: Good question. I have no idea.
My guess regarding Roderick: Roderick had dementia. I assume he wasn't thinking clearly, and thought he could avoid Verna's wraith. He thought he could make Madeline immortal by mummifying her.
It's obvious that they were not dead yet and those were their "rage against the dying of the light" final moments.
By the end, Roderick knew both of them will soon met their "the end of the Usher's bloodline". He wanted his sister to die painlessly and have an afterlife like Pharaoh. We don't see Madeline vomiting blood or foam before passing out, so it is unclear whether she was poisoned or drugged. Ironically, the mummifying process performed by the very sick and delusional Roderick didn't finish her properly but made her dying in agony instead.
Do you think Madeline got what she deserved as the mastermind of two of them?
Man, I am not sure.
It was also alluded to that Madeline had some sort of extra powers when she was talking to Verna, but nothing came of that. Now how could Madeline see how to get up stairs and strangle her brother if her eyes had been cut out and replaced by those gems? And back to the mom, I dunno, she seemed like some undead creature that barely recognized her children. I guess not everything requires a detailed explanation to be enjoyed.
The noise and the voice: During Roderick's confession, both of them keep hearing noise from the basement. Which could be interpreted as blind Madeline was struggling to get up stairs. At the end, she was able to strangle Roderick by following his voice. Which was extraordinary but not beyond natural.
The dialog and the begging: After the mom crawled out of the grave, of course she looked like an undead and was in full defense mode. Who wouldn't? Once she learned that it was an accident and that they didn't mean to bury her alive, she stopped strangling the boy. Before that moment, what the sibling said to each other and then to their mom made it clear that none of them thought their mom was "undead creature" who "rise from the dead to kill".