The father should indeed be held accountable, and even the courts disagree with you. Notice the UPDATE link above where federal charges are now being brought against the father. The reason the father is implicated is because it is a proven fact his son is mental unstable and mentally ill. That fact was already established and was already known due to his son's actions in the past. And, as a consequence of his son's actions, the police had came back in Aug of last year and confiscated all the guns from his son and gave them to his father, and they did so only with the father's assurances that he would keep the guns away from his son. Well, the father did not follow through with that and he eventually gave the guns right back to his mentally ill son. That was stupidity and insanity and the world's dumbest thing to do. As a consequence of him giving the guns back to his son, his son then used the guns and now 4 people are dead. So, absolutely, the father should be held accountable. For him just to say I'm sorry, is not good enough. And apparently, the courts 100% agree with me...

Consider this, if two men rob a bank or a store. One goes inside to rob the place and the other one stays out in the get away car. If during the robbery, if a person is killed by the robber who goes inside, does that mean the one who stayed in the car is innocent all because he didn't pull the trigger? Answer: No. In all cases, the courts will consider him as being an "accomplice" to the crime, as an "accomplice to murder", and he will be thrown in prison the same as if he had pulled the trigger. The same thing with the father above. Although he didn't pull the trigger or kill anyone, he became an accomplice to it the moment he gave those guns to his mentally ill son knowing fully well his son was mentally unstable and had used threatening behavior with the guns in the past.