Plausible deniability for what???
For not being psychic enough to know the future?? The killer is dead. We aren't the kind of people who blame the weapon, instead of the shooter, so what's left, blame the family who "knew"? Or blame them because they "should have known?", and didn't turn him in??
Things aren't that simple, and its very, very easy to judge, AFTER the fact, and from outside the situation.
His Uncle said "never in a million years" would he have thought his nephew was capable of doing what he did.
People often show one kind of behavior in public, and to their family, while having something entirely different in their hearts. Even in cases where someone states (sometimes repeatedly) that they are "going to kill that so and so" it is very seldom that such threats are taken seriously.
Also remember that there are two sides to every story, and while the side being reported on today is "he had a history of domestic violence" and therefore was a bad person, the side the family might have heard could have been he was a victim of the system, who got in a fight with his wife, one time, and the system railroaded him, etc.
The information we are seeing about him is being shown to us by investigators, people who find the pieces of the puzzle, and put them together to show the real picture, I find it entirely plausible that while the family might have had all the pieces, they didn't have the ability to put them together.
I read both the linked articles, and a couple of things in the Washington Post article bothered me a bit. The big one was that they said "After fleeing the scene, he was confronted by at least one armed resident and took his own life soon after, police said."
"After fleeing the scene" is quite different from what others are reporting. The other thing that bugged me might just be my opinion about proper journalistic style, but saying "at least" one armed resident, and later saying that police recovered "at least" 4 guns, simply tells me the author doesn't have verifiable FACTS, and is just writing to hear themselves speak, not to pass along factual information.
The investigation is ongoing, there may well be more pertinent information disclosed. I think that blaming the family, or the background check system is the wrong approach. The person responsible is the one who pulled the trigger, who is currently dead, and appears will remain so for the foreseeable future.
I do think we need to point out to people that an "AR style" rifle was used to STOP the attack. That they aren't just the "weapon of mass murders" but they are also the weapon of the defenders, as well.