Ha, yeah even as a kid who was a big believer I never really understood why an apple from a "Tree of Knowledge" was somehow such a bad thing.
Well, it's the tree of knowledge of good and evil, so I guess the problem is that it unleashed evil into the world. But as I see it, good and evil are choices people make. Knowledge brings the power to choose.
Although it's interesting to look over
Genesis Ch. 3 and see how the story unfolds. Eating the fruit doesn't really do anything except make Adam and Eve aware and embarrassed that they're naked, i.e. create a sense of shame -- and shame can be a powerful incentive to avoid doing evil as society defines it. So they didn't really do anything wrong except eat from the tree, and for that God banishes them and curses them with stuff like labor pains for women and the hard labor of farming for men. And then God says "The man has now become like one of us, knowing good and evil." In other words,
God knew good and evil -- i.e. had the capacity for doing harm to others -- before humans did. That was an aspect of godhood that humanity gained. So it's not exactly saying that God was a force of pure goodness. I guess that's an idea that came along in the New Testament; deities in earlier traditions tended to be pretty morally ambiguous and capricious.
(Also, "like one of us?" Who is God talking about/to there? Genesis was basically adapted from the Babylonian creation myth
Enuma Elish with the polytheism edited out, but maybe this bit slipped past the censors.)
Basically, though, the gist is that Adam and Eve were like animals, innocent and carefree, and then gained the godlike power of knowledge and choice, becoming fully human with all the good and evil, and all the responsibility, that that entails. And maybe the idea was that we got that bit of divine power before we'd earned the right to be trusted with it.