Certainly no blame can be attributed to the father. He did nothing illegal and his son was a grown man who should be responsible for himself.
What I will say though is that if you bring a child up in a household where guns are considered acceptable, even necessary, that child is far more likely to be willing to use a gun than a child from a household which does not own guns and considers them unnecessary and dangerous.
There is quite a common thing in the US, when asked about why they need guns, to hark back to the formation of the country and the need to defend themselves from tyrants. So if they consider someone a tyrant isn't that the entire point of having the guns? It's a nonsense but if you bring a child up with that kind of mindset during their most impressionable years, it must increase the risk that they'll do something about it.