Sam without Dean was a man on a mission: dark, uncaring, unfeeling. He was Justice personified,untempered by compassion or love. His only goal:revenge–finding the Trickster and getting Dean back. It was an obsession that controlled him, burned inside of him, and fueled his every action. This was Dark!Sam, all humanity, everything that made him a person, seared away by the fires of obsession, just as the fires of Hell burn away all that is human, creating demons
from those unfortunate enough to land there. What we knew of Sam was gone, replaced by
someone whose life had only one direction, one purpose.
—  From the supernatural book “In the hunt”(published just after s3 finished) pg 244-245