He attended the University of Louisville, and denounced it as making him too white. Greenberg is a former board member of the university. So that is a possible motive. But among all the board members, university admin people, and professors in the area, the would-be assassin chose Greenberg ... because he's Jewish? These potential, non-exclusive, motives have been out there as possibilities from Day 1.
Supposedly the guy has "mental issues". Whether or no, he was sane enough to select, out of 10s or 100s of thousands of potential targets, a Jewish guy with ties to the university the guy hated, and try to assassinate him with a real gun instead of a feather-duster. IMO, being able to plan and select a target means the would-be assassin is sane enough to be criminally culpable.