Second Foundation

Many years ago I was a great Asimov fan. I read a whole lot of his stuff, but I don't remember much of it, so I reread the Foundation trilogy. The thing I realized all those years ago is that Asimov's short stories are better than his novels. That holds for his Foundation trilogy. It was good, but not great, After rereading it I'm kind of surprised that it has garnered so much acclaim. perhaps it was that it the first of its genre.

I thought the second book, Foundation and Empire, was the most engaging of the three.

I'm left with this question: In the third book, Second Foundation, the people of the first Foundation consider the Second Foundation to be their nemesis. This is the whole basis for the story. Hari Seldon had created the Foundations, both the first Foundation and the Second Foundation, to be the watchmen of the Seldon Plan. With the rise of The Mule in the second book, Foundation and Empire, the Seldon Plan would have unraveled, had it not been for the Second Foundation. As far as we are led to believe, the Second Foundation is benevolent. Why does the first Foundation fear it so?

I wondered this all the way through the third book, expecting that sooner or later it would be made clear, but it never was. Did I overlook something? Why was the Second Foundation thought to be a malevolent force?