Perhaps you should wait for the end of the series before commenting on the characters' black-and-whiteness. Most characters, after all, are only children or teenagers, and not yet complete, fully developed personalities. Plus we look at things from Harry's perspective and yes, he sees things in black and white in Book 4. I wonder what you were like at 15? Were you as wise as you are now? The series shows us Harry's progress - it's a coming of age story, in a way. Wait till the end of the series to see how he ends up, before you bash it.
The Golden Trio keep suspecting Snape for a long, long time, especially in Book 1, despite Dumbledore's trust in him. It continues through all the other books, but in the end it's revealed he was not a bad man, actually. Not a good many, either, probably - but then again, it's not easy to determine whether someone is good or bad, most of us are somewhere in the grey area between black and white. Voldemort's childhood is explored in book 6 and we discover that he was not bad because he was bad - circumstances 'helped' him become 'evil'. Another example is Draco Malfoy, who Harry sees as a bad person and an enemy in books 1-7, but is actually a misunderstood young man, a result of his upbringing. The series begins with strong prejudices about pupils of different houses, but as Harry matures we see that there are 'good' and 'bad' people in all houses.
So please, do not be impatient. Book 4 is only half-way through Harry's development into an adult an it's undestandable he still has a long way to go. I think it portrays a 14-year-old's perspective brilliantly!