I have an off-the-wall question. Since he killed people, and he blamed his parents for ignoring the signs of his social deficits... can they be held responsible for the deaths since he is now dead and he cannot be held responsible?
Even though their son was not a minor, they could be named in a civil suit, but whether they’d ultimately be found liable would depend on multiple factors: knowledge of illness, knowledge of threats, access to weapons, etc. Again, this is civil - not criminal.
You might look into Cosmo DiNardo’s parents, Phila Court of Common Pleas. There, the allegation is that the parents knew of his illness and failed to secure access to their firearms. The son was around age 24 at the time of the murders IIRC. (The mother did then seek indemnity from the son’s MDs; that suit was denied).