I don’t know how much more we’ll learn about the shooter’s motivations. Right now he’s lawyered up, so maybe he won’t volunteer any more incriminating information. But perhaps the police will find incriminating statements on his computer files.
One thing I’d note is that neuroscience has a very reductive view of human nature. In Daniel Dennett’s catchphrase, “Yes, we have a soul, but it’s made of lots of tiny robots.”
If Holmes imbued that philosophy, then it might explain why he places so little value on human life.