I don't like it either that they made Dean do it. And yes I remember Leonore (how could I not?) but then Sam was there. Dean got there because Sam was captured and because Gordon went off the deep end like that. Now it's more like Dean doesn't know if Sam's judgment is a good one because of what's going on in his head. Also, Amy did kill people to save her son, what else would she do in the future? Leonore killed cows. I know it's not a good comparison. But in some ways I get why Dean did it. Only, the character development is kinda moot now. It is weird, mostly because we don't know what Dean's really thinking these days. Everyone around him is unstable so he kinda flees into the one thing that was always there, kill the monster no matter what.
The lying to Sam thing bugs me the most about this.

Bobby was so cheerful this episode, so lax about things. I don't know.. that was weird.

It was a good one, yes. :)