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Another fun aspect of the season was the resurrection of Grandpa Samuel Campbell. “As we know, the Campbells are a long line of hunters, which is a somewhat unusual situation,” notes [Sera] Gamble. “Hunters are often created when a personal tragedy happens in a family, like Gordon, who became a hunter when he saw his sister killed. The Campbells have been hunting for many, many generations, which is an idea that has intrigued the writers for a long time, ever since we figured out that in the Mary episode [‘In the Beginning’]. The idea is that all the civilian Campbells are dead – Yellow Eyes killed them – but the active hunter Campbells have managed to lay low and only work with each other. They’re monster specialists, and in this landscape where the monster situation is increasingly worrisome, they are now working together. Samuel Campbell is part of that story. Obviously there was a lot of mystery around how Samuel would be there.” Until, that is, it was revealed that he was in league with Crowley, capturing alpha monsters for the demon in exchange for the promise of resurrecting Mary. Then he acted somewhat soulless himself, or at least heartless, when he left his grandsons to be eaten by ghouls in 'Caged Heat.’ “Dean wanted to kill him,” Gamble says, “which is a big deal. If you had stopped Dean in the street two years ago and asked, 'Is there a situation you can think of where you would kill your own grandfather?’ he would’ve just laughed in your face. But the concept of family has really been altered for Dean this season. It’s much less about blood than it ever used to be. He was assigned Sam from a very young age – 'He’s your brother, you have to take care of him.’ – and the bond went from there. But the people he aligns himself with now, including Sam, are much more of a choice and have a lot less to do with blood and genetics than they ever did before.”

Source:
Knight, Nicholas. Supernatural: The Official Companion Season 6. Titan Books, 2011: 9-10. [gif]

tl;dr