Last year, WS allowed a video made by a former classmate, Sloan.
In it, Sloan describes how easy it is to enter the building without a problem. Security cameras were not working; some still covered in plastic they were so new. The AVA buildings housed 1000 residents. Dispatch said he was bleeding from his mouth, hands and neck with trauma to the chest.
My current theory is that it may have been someone with a grudge from his college, in general. In particular, perhaps it's a male from the Frat house or someone denied joining SChi or another type issue w/the frat, in particular. He served as VP of the Sigma Chi frat during his time at University.