Here's the thing: you're never going to get a satisfactory answer that explains why Cho Seung-Hui did what he did. How could you? What is the reasonable explanation for why a run-of-the-mill college student decided to walk into a building full of strangers, chain the doors shut, and then gun them down? There isn't one.
What you have here is a strange, seriously disturbed individual who frightened some of his fellow students and teachers with his writing and behavior. His behavior and his thinking were not normal and so you can't reason them out normally. Were he still alive, perhaps you could sit him down with a psychologist for a few months and tease some sort of twisted rationale out of him, but it wouldn't make sense -- not in the way that we want it to.
In a situation like this, we want to know the exact events that set the murderer off so we can stop people like Cho Seung-Hui before they kill again. But, in the spiderweb of a mass murderer's mind, you never know what sort of muddled thinking and bizarre associations may be present.Seung-Hui could have been upset because he fantasized about a girl and she didn't give him the time of day, because of bad grades, or even because of a misinterpreted remark that the average person wouldn't have thought about for two seconds. On the other hand, despite all of his planning, if Seung-Hui had a happier week, he may have never gone through with his plans to murder those students.
The reality is that we're never going to really understand the "why" behind Seung-Hui's actions any more than we truly understand why Klebold and Harris murdered kids at Columbine, Ed Gein had a necklace made out of human lips, or why Ted Bundy eventually chose to rape and murder his way across the country.
You can chalk it up to evil. Psychologists and profilers can study these cases to see what they can learn, but don't expect it to really ever make sense because there's not going to be a rational, logical explanation behind it.