Cho was a very silent guy during childhood, but went to school and managed to perform well in school. His detachment justifies a Schizoid Personality Disorder diagnosis, premorbid to Schizophrenia, Paranoid Type that presented at Virginia Tech in 2005, perhaps even earlier, in his early 20's. A very common onset for schizophrenia.
His final words, "But you decided to spill my blood. You forced me into a corner and gave me only one option. The decision was yours. Now you have blood on your hands that will never wash off."
To me that sounds as if he thinks he's the one who had been crucified 2 millenium ago (I'm admittedly no expert on this), a sign of schizophrenia, thinking you're the father of Christianity.
We still don't know if he was an abused child. It has been repeated in the news that his parents worked very hard and stressed achievement, which explains why he and his sister tried and did very well in school, despite whatever stressors they suffered. CNN quotes someone as saying that Cho's father said that he had to leave Korea to come to America "where no one knew him."
That sounds suspiciously like foul play somewhere, somehow. In any case, Cho's mental illness surely could have been flagged much younger.
My guess is that therapy docs will be seeing many very quiet children in the next few months, perhaps years, and that's for the best if you ask me.
I like quiet kids. They have a lot to say once you get them talking.