"Are you sure that what you brought back is 100% Sam?" The demon Azazel taunted Dean at the end of Season 2 after Dean brought Sam back to life. We see the cold hatred in Sam's eyes as he kills Jake, and he slowly starts acting less like himself over the course of the next season. Sam has always been known as the one who values human life, but it seems any time either of the boys are brought back to life, they lose some of their Humanity.
Do you leave something behind when you die?
In the first season when the boys realize demons are real, they do all in their power to save the victims of the demons.
They use holy water and exorcise the demons, valuing the human inside. Over the seasons, however, the boys stopped caring as much about saving the human, and have been more concerned with killing the demon. By the end of Season 11, Dean died at least 106 times, and Sam died at least five times. This could explain why Sam is usually more aware and merciful.
Others who may be proof
Any time someone dies, could it be that they lose a little part of their humanity? Their father, John, was killed in episode three of season 4, and was brought back to life thanks to a deal his wife Mary made with Azazel. He is gruff and very strict with the boys after Mary's death, and this could have been brought on by his resurrection.
Their mother Mary is killed by Azazel in the very first episode, and was resurrected by Amara in the last episode of Season 11. She is more efficient in her new life, doing whatever it takes to keep her sons safe.
Many others have died and have been resurrected, and haven't quite been themselves. Dying in any way may cause a character to lose a portion of their humanity.
The only conflicting person to this theory may be Castiel, the angel. He has died many times, yet he seems to be gaining humanity. He has compassion for humans and strives to be like them. It may be that humans lose parts of themselves, while angels gain more humanity every time they die. In the first episode that we see Castiel in -- episode one of season 4 -- he is an almighty warrior of God.
Slowly, with the help of the Winchester boys, he becomes more human and more merciful.
It may be that anyone who dies in "Supernatural" is significantly changed when they return to life. Dean and Sam are possible examples of this, as are their parents and Castiel. Somehow, passing through the veil of death seems to trap part of themselves within it, and the more deaths you have, the more you lose. Hopefully seasons 12 and 13 will provide a clearer view on this.
