Sam has not always been a curmudgeonly old nasty dog. He used to be quite easy going, and as far as dogs go was about the best you were going to get. We are not dog people at the best of times, but we have sort of gotten on with the dog-dad looked after him most.
My own theory is that dad in his estimation was the omega (bottom dogson) of the family and now that dad is gone, Sam knows he is the omega. As nobody wants to be the bottom of the pile, he is behaving aggressively and being wilfully disobedient in the hopes of challenging us for our spots in the family and having at least one dogson to look down on. When he does it of course I have a tendency to see red and start beating him with whatever is convenient, especially when he starts snarling and/or snapping at me. Mama is just as bad, and so he is always making trouble for himself.
He used to be pretty good natured. He let me dress him up as Osama Bin Laden:
And he let me put him in a cart and make him pretend to be a postal employee (though you can't see it he is wearing a postal tie) :
And I used to play fight with him.
Now I am usually just fighting with him. My SIL said that on the farm they often had dogs that got mean and ugly in their old age. I don't know if that is Sam's problem or if he will get over being his Majesty the Prick of Tails. As we will never get rid of him until he dies of extreme old age, I rather hope he does.