James Holmes -- Obsessed with NON-VIOLENT Video Game Guitar Hero

The man who slaughtered a dozen people and wounded 59 was obsessed with a video game that was anything but violent ... Guitar Hero.

We've learned ... one of James Holmes' classmates at the University of California at Riverside played Guitar Hero with the shooter all the time, and "for hours."

The male student is telling friends he is "shocked," never having had a clue that Holmes had a propensity for violence.

It's an interesting side note, because people with violent tendencies often act them out in various ways, and video games are definitely an outlet.