If the weapons were not secured, I hope so.
Knowing that somebody who was residing in their home was mentally ill*, forbidden to own weapons, and then had been acting erratically with one (firing a shot gun on a city street), any weapons owned by the parents needed to have been very secure.
My bet is that they were not. If that is the case, they need to face criminal charges. Sadly, I don't think secured weapons would have stopped these crimes as the perpetrator appears to of had access to illegal weapons through the drug trade. That, however, is a separate matter.
* Though the perpetrator may have a few "screws loose", he still had enough presence of mind to plan the murders and then try to conceal them. In short, he is responsible for his actions in my opinion.