It's common in fur farms that they breed the animals that are most friendly to humans, and in only a few generations those animals behave like very friendly pets, and killing them becomes more difficult.
A NOVA episode on dogs showed a Russian study where they bred the most friendly foxes with each other and did the same with the most aggressive (so extremes in both directions). The aggressive-bred animals were like that scene in I Am Legend where he checks on his infected rats. They were being fed, and they still wanted to kill their feeders. Kind of terrifying.
