I wonder if it could have been say, a member of the Latin Kings or similar, who was at either the bar or the frat house, and was rejected and had his ego crushed? I say this because they are known to be quick-tempered and violent if offended and/or their turf is crossed, and the kind of violence displayed here is indicative of someone like that (or a serial killer). I can't think of someone who is otherwise 'normal' losing his chit in a one-off crime of this type and magnitude.
In my view, this was not perpetrated by someone who hasn't yet had the kind of life experience to be so sadistic and depraved. I am not thinking a college kid, even if troubled, could have such developmental outliers at this point in his life. Not impossible, but unlikely.
My opinion.