Thinking out loud here.
We know that the poltergeist pretty much hates our guts. After speaking to Purvis, I think I came up with a few possible reasons for this, and they all happened before we were cast into the void:
1) The part where we threatened to drop the poltergeist if she struggled. This may have made her think that we were strictly malevolent, if she didn't already. Though she was glaring at us even beforehand, so this probably wasn't it.
2) Taking her out of the forest in the first place. That's where she was hiding from everyone, so being forcefully taken out of her element was have been a very negative thing. Though this couldn't be helped, because we needed to find some way to get the shadows back.
3)
"Many others already know all that I do. Can you track them all down? This doesn't end if you stop me."
Even if the poltergeist was able to stay in the forest, this line would have told her that someone was out spreading information about her. This is probably the thing that set her off the most. ...Though she was already attacking us at the time, so...
If nothing else, I think this whole situation has been approached the wrong way. Some of it probably can't be fixed, but there has to be at least one thing that can. ...If I were a more analytical sort, I might be able to help more, but as it stands, things don't look good.