in my opinion it would be interessting, if the co-pilot took any medical drugs like antidepressants. Sadly most spree killers was on antidepressants. I think it is very likely that he took those drugs.
This would be an possible explanation (for me) because i cannot understand or believe, that a well educated and intelligent person would do such a thing.
He was a young guy, smart, had a good job... it make absolutly no sense...he must be on drugs or something in my opinion.