are dean and cas in love? (read the full analysis here)
‘Set aside all the killing people or not killing people or whatever, and you’re left with this: Amy is a supernatural being that Sam fell in love with as a child. More importantly, from Dean’s perspective:Amy is a supernatural being that Sam once fell in love with when he was too stupid to know not to. Dean knows that no matter how innocent she seems, Amy is a snake in the grass. She’s dangerous. And by killing Amy, Dean thinks he’s doing Sam a favor. Sam’s blinded by nostalgia, Dean thinks, and so it’s up to Dean to do what must be done.
This is a pretty obvious projection on Dean’s part of his feelings about what Cas did to him. Cas seemed innocuous, and Dean trusted him. But then Cas broke Sam’s wall, opened the door to Purgatory and unleashed the Leviathan. Cas, by virtue of being a supernatural being, is and always was dangerous, and Dean forgot it. He let his feelings cloud his judgment, and now everyone must pay the price.
Here’s why I really emphasized all that stuff about A plots and B/C plots earlier, because without understanding how the A plot mirrors the C plot and vice versa, you’re missing out on what this episode is really trying to tell you.
Basically, the A plot in this episode (the Amy storyline) acts as a reflection of the C plot (Dean’s grief), in which Dean realizes that Cas is the supernatural being that he fell in love with when he was too stupid to know not to. And so by killing Amy, Dean is attempting to retroactively fix that mistake with Cas.’