This is so classic for these kind of shooting sprees. LONER, quiet; heavily dependent on girlfriend or other person.
would rather see her dead than with someone else Then decides he's going to kill a bunch of others out of anger. (Charles Whitman)
It's classic. He's not a Jihadist ; he's a nutcase. What makes him different is that he's from South Korea; although there are plenty of Asian murders, we had one here in this town not long ago, but he murdered one person, his teacher.
This kind of mass murder can run through all walks of life, differing nationalities.