News reports are claiming the mother told police the son has a history of mental illness.
Mental illness isn’t a defense. It would be an explanation for the social isolation, the extreme behavior, and the need to take 6 years to graduate, when finances were not a factor.
Had he killed his father in the heat of the moment, sure, I could find “spoiled brat” to be a sufficient explanation. The crime as described is cold-blooded murder, without any effort to get away or to escape blame. “Go get me a sandwich Mom,” then return home to barricade yourself in your apartment is not a plan. It’s also not a sane plan to inherit a fortune, as a murderer doesn’t profit from his crime.