It would be a great if you give the link of the news, since the incident has attracted world people and me to know more about it. I still don't get a complete reason why he did that, maybe he is hiding something as the main motive.
The details are still emerging, but it seems that he suffered from mental illness for quite a long time.
And regarding the motive, I believe that it is clear now. He told the victims himself that he was doing it, as a retaliation for rape of white women by black men. And the following article makes it clear that he was a neo-Nazi: