EDIT: I'll try to be clearer in what I want to know.

The philosophy of good/bad, like/dislike is really irrelevant. 

Question: If karma is neutral, then how come "good" karma leads to "good" rebirth. These aren't my words, I've heard it being said: "good karma leads to good rebirth". What "good" means in the context of Buddhism is beside the point and why Buddhists even use the term "good rebirth" is another question. Weather or not being rich is ultimately a "good" thing, "good" karma still leads, rather conveniently, to "good" rebirth, which I take as meaning rich and healthy since that is the only interpretation I've heard.

But what I want to understand is why doing good can't lead to rebirth as, for example, a worm? Why does "good rebirth" mean to be born generally fortunate (by unenlightened standards) when karma is neutral?

I hope that clarifies.