Why did you need to set yourself as the authority on what was best for everyone and demand she end communication with others?

Because she was my friend first and they were extending me a courtesy by making an effort. I'm not the authority, I'm just responsible to them.

And the way in which she was toxic is not in a way that is immediately clear to others that she is toxic. A premise which you surely understand, because you seem to ascribe it to me, with people on my side even though you think I'm wrong. I suppose I could have explained it all to them and left them to make their decision, but quite frankly, I'm sympathetic to the way she is, and didn't want to make her out to be the bad guy to the people in the group. I held out hope, for longer than I probably should have, that I could convince her to back off so she wouldn't be so toxic, so she could get something out of it.

It's also clear that you intend to continue to "punish her" by withholding information for a guy who likes her.

I didn't decide to withhold information. I'm respecting her choice to withhold that information. I didn't tell or suggest she do anything of the sort. In fact, my preference was to go to him and explain the whole thing, and she asked me not to. I've been respecting her request. I came here, in part, to ask if that was a good idea.