Episode 7.03 - The girl next door [I didn’t get it]

Rewatch Supernatural at the moment and saw 7.03 yesterday—and it still keeps me thinking.

First of all: I loved this episode. I loved to see Mini Sam again and how he first met Amy.
It’s an episode about moral decisions and grey linings.
But I don’t get why Dean didn’t trust Sam in the end and decided to kill Amy anyway. It doesn’t make sense to me. A couple of seasons back he let the vampires go who only drink animal blood (but I’m pretty sure they drank human blood before). I don’t get the difference between these two decisions.

(Spoiler for everyone who didn’t watch the whole series yet)

!And why did they bring up the “Boy will only kill Dean someday”? As I remember he was never be mentioned again. Did the writer forget about it? It would be a great ending if he would have killed Dean in the end, but I think with “god write all the stuff” it sadly didn’t make any sense anymore.!<

What do you think?