We had a German shepherd when I was a kid, Fritz, great dog, you could fight him on the living room rug and he would snarl and almost bite you but not quite. He was definitely the alpha male dog in the neighbourhood but as far as people went, if we liked someone then he did too.
This was back in the days when milk was delivered to your house, and bread too, and even dry cleaning. So the dry cleaning guy came once a week, the dog would wag his tail, no problem. Then the dry cleaning guy got sick, his replacement knocked on the door, and when Fritz saw him he went full ballistic fucking apeshit, he hated this guy so much he was barking and foaming at the mouth. He genuinely wanted to kill the guy, no question, and it took all of your strength to hold him back. This went on for a few weeks, then the regular guy came back.
A couple of months later my Mom is reading the local paper and she says "hey look at this". In the courtroom reports there is a mention of the substitute driver, who got 90 days or something like that for passing bad paper. Fritz just knew instinctively that the guy was garbage.