When it comes to the parents, I guess I'm in a sort of "society is to blame" mode.
I see a couple of general themes among people who want to prosecute the parents, including the actual prosecutors.
First, there's the "they should have known" theme. I don't disagree with that, but if we locked up all the bad parents or even just the parents who were unaware of or just underestimated their kids' mental health issues, the jails would be overflowing. Did your parents really understand you as a teenager? Mine didn't. 40 years later I was still rolling my eyes as my mother would explain what happened to me in my teen years, and get it so incredibly wrong. I thing parents are among the absolute worst judges of their own children's issues. Yeah, they were bad parents, but I'm not going to lock people up for that.
Then there's the access to the gun theme. That's where I really go with "society is to blame". These morons were doing what they have been encouraged to do by a very large segment of American society. We have congressmen posing with their guns on their Christmas cards. Our society is insane. What these parents did, with keeping an unlocked, loaded, gun in the house, ought to be illegal. Sadly, it isn't. Can we prosecute the Republican Party for manslaughter?
So, what's left? I won't prosecute them for gun access, because it is legal. As insane as that is, it's legal. I won't prosecute them for being bad parents because there are lots of bad parents. And...who cares what I think, but I think the prosecutor will have a hard time finding a jury that won't think the same way. But, there is one other angle that might work, with me, and with a jury. If the prosecution can show that the parents actually did know of his mental health issues, and concealed that information, that's a different story. Likewise, if the prosecution can show that the mental health professionals recognized the danger, and sought information, and the parents concealed it, then you have a concrete, affirmative, act that a case can hang on. I think that's their task, but I'm not optimistic they can achieve it.
Believe it or not, I want the parents to be guilty. However, more than that, I think the rule of law is important. People should only be prosecuted based on what the law is, not what it should be. I also really dislike the trend of interpreting the law in a way that is so broad that it catches the most people in the net. I think the opposite should be true. In order to get a conviction, people should have to violate the plain provisions of the law, not the most expansive possible interpretation. I look at the concept of manslaughter, and I see a very high bar before it can be used to prosecute neglect or stupidity.