Yet even here, in the most black and white of moments, fate insists on making Dean see the gray. In full killer mode, he looks up to see a terrified young boy, something which always triggers his protective instincts. Clearly, he can’t protect this child, having just killed his mother. He cannot kill him, either. Dean allows the boy to live, collapsing the black and white world view which he has been clinging to for stability.
This episode caused some upset in the fan community and with some critics, because Dean appeared to be reverting to a black and white insistence on what defines a monster. I think the writers are playing with Dean’s definition of monster quite deliberately. Having collapsed the dichotomy Dean was relying upon to justify his actions to himself, he’s still left with the fact that he killed Amy and lied to Sam. He can’t change what he did and he’s not sure he should even if he could. Does this make him a monster? Dean is very aware of gray areas. He just doesn’t know how to live in them.
And that leads Osiris straight to Dean. Despite Sam’s spirited defence of his brother, he misses the real point. Dean feels responsibility for taking care of anyone he defines as family. Yet the very nature of his job means he puts people in danger every time he interacts with them. Jo and Sam can tell him not to feel guilty until the cows come home. Dean is very aware of the ugly side of his job and he feels he ought to do it alone, so others can be untainted.
That feeling is what led him to lie to Sam and kill Amy on his own. Far from feeling no remorse, he knew this killing would feel awful. He decided he had to do it anyway and chose not to drag Sam into it, as Sam had such a debt to Amy. He allowed Sam to stay untainted—or at least that’s the way he saw it at the time. Now he feels the full weight of how gray that situation really was, which makes him even more reluctant to open up to Sam. The last thing he wants to see is judgement in his brother’s eyes.
Dean’s guilt over Amy is Osiris’s trump card. Sam may be able to argue convincingly that Dean is not responsible for the bad things in his and Jo’s life, but would he even try to do so about Amy’s death? Dean feels the answer is no and he chooses not just to continue lying, but to accept death rather than risk his brother’s love.