Samantha didn't change at all. She was always scheming and volatile!
When we first met Samantha, we simply didn't know her well enough to see those qualities lurking beneath the surface. And I'm sure that the grand lady of Collinwood didn't want people knowing the darkness that lurked beneath that well-lacquered veneer of gentility and moral uprightness.
Of course, we did see hints of Samantha's true character in her exchanges with Gabriel. At the time, it looked like she just didn't like him in particular. It was only later we learned that she was basically a bitch to everyone!
It's a testament to Virginia Vestoff's talent that she gave Samantha such life and believability - such a good actress! The writers should be given their props too. The character of Samantha is just so multi-dimensional, and proves that 1840 isn't just a shoddy retread of 1897.
(As an aside, it's a little surprising that the mental asylum allowed patients to roam freely on the edge of a cliff ... )
I thought Samantha threw Joanna off the cliff on Collins property,
after the latter had been released from the asylum. I was pretty sleepy when I watched that particular episode, so I could be wrong.