In answer to some of considering motive: <modsnip: stating opinion as fact>
His general personality has been described by friends and clasmates as severe outlier in ego. There was some kind of event at his school shortly before, where undergraduate students he was grading , felt he had been overly critical in his remarks on their papers and given them unfairly low grade. Some kind of semi-public dressing down by his academic and intellection "lessors" occurred, which given possible ego issues, could have been profoundly humiliatingly, and put him over the edge. he could have projected and focused his anger on any undergrads, especially if there were undergrads he knew who had, in his perception, also humiliated, rejected or insulted him. he could have been ideating killing for a long time, and some set of events set him off