To avoid spoilers of the end of the movie, I reduced the title, originally I intended to ask:
Why did Thomasin give in to Devil so easily
Which is the real question here - at the end of movie, I expected here to try to get revenge. I thought she will try to stab Black Phillip. Of course, killing a goat doesn't kill the Devil and would probably lead to her demise. But it would be expected flow of events.
Instead, she suddenly turned the sides. Why? Was she the Witch all along, just not aware of it herself?