Cho returned to his dorm room after the first shooting to regroup, reload, prepare, write a note and search the university web site for his next victim. Reports indicate Professor Herr Bishop was his next victim. What grievance Cho held against Prof. Bishop is unclear. Perhaps bad grades. Perhaps Cho was planning to go for his Master's, and Prof. Bishop was an obstacle.
The receipt for the gun purchase was left in his backpack inadvertently. I believe Cho forgot it was there. He had defaced the serial no.s to create a "drop piece." At some point Cho must have planned to use his weapons for some "hit" he would survive and that would involve discarding the gun(s). That is why he tried to make them untraceable.
Will be interesting to learn when Cho purchased additional magazines, and he must have had plenty. At that time Cho changed his plans from conducting an assassination operation he survived to a suicidal mass murder.
I don’t think any of the profs were specifically targeted.
The whack job was looking for his ex-girlfriend and her new lover. Supposedly they had classes in the Norris building.
Mostly, the profs were trying to defend their students. Excellent brilliant men, killed at random.