I think that we have to accept that in the world of Tolkien giant spiders are evil !
As statet before Ungoliant was greedy, she wantet to eat all just because she could. It was this that became the end of her.
Shelob is called evil in the books is she not ? She like Ungoliant want's to eat everything she can. (- Gollum as he could be of some use) If we do not accept that at least these two are evil, we will end up with a philosophical discusion of what evil is.
Or at least a confusing story, becouse then the orcs are not evil, they do not realy want to fight, neither do the mouth of Sauron. He just wants a nice place to live
