Where did you study to get your military psychology degree? Because just by logging onto the internet and reading a news report without having any military personnel records, medical records or even knowing anything about that person, you seem to have come to a very professional diagnosis, I'm surprised you haven't been headhunted by the ministry of defence.
I am sorry, I don't mean to sound disrespectful and i'm also sorry to hear about your uncle i can't imagine the torment and upset he had to live with, as post dramatic stress from military service is a terrible thing to suffer with, however as you rightly said everyone reacts differently
There is still an investigation going underway and the soldier in question handed himself in alive for questioning. Only he knows why he did that, and it really could be as simple as having his best friend killed in front of him and red mist took over, because some people really can just snap like that, with no prior history of a mental build up. but we don't know, anything on at this point is only speculation and we will only know what really happend after he is questioned and examined by a psychiatrist.