Status Updates From Supernatural and Philosophy...

“It is a great good to allow creatures like humans to make their own choices, and one of the consequences of allowing for this kind of choice is that, on some occasions, evil will be treely chosen and produced. If this solution works, God could prevent evil, but to do so would be to eliminate free will”
// In that case I wonder how many (or how few) people would actually renounce their free will for world peace?.

Mary makes a deal with a demon for John's life; John makes a deal with a demon to save Dean's life; Dean makes a deal with a demon to save Sam's life; and eventually, Sam just ends up in the cage with Lucifer anyway. […]
No doubt […] as a result of these deals, the Apocalypse was averted. But there may not have been an Apocalypse in the first place had Mary, John, and Dean not made their deals in the first place.

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Whether one views family loyalty as a strength or weakness, it is the most significant moral guidepost in Sam and Dean's lives.
Above everything else they value, it exercises the greatest influence on their will. Given this influence, we should wonder how free this will really is.

The short answer is that most of us are "speciesist." We are prejudiced in elevating the human species over other species in terms of rights and values. We keep animals as pets, and many of us readily eat, wear, and sit on the flesh of other species. In many cases, monsters just treat humans the way we treat other animals.
We fail to see the symmetry, though, since we place ourselves above all other species.