who do you think the last person was that Dean was feeling guilty about in the last episoide?

I think it was Amy. And he didn`t want Sam to know about it, so he stopped the trial before the last witness.

I definitely think it was Amy. If it was Cas, I don't think he would have cared if Sam found out, which was the whole point of stopping the trial before the "final witness" showed up; so Sam didn't know who it was.

if not because dean thought about amy that osiris thought about the same witness...maybe he's thinking about adam who's lost in the cage...don't think of john because in the end of the season 2, that was like a peace goodbye for all the winchester characters...about castiel, that would be great because jensen and gamble're talking about the guilty of dean about castiel..but nothing like always...we wait about a character but dean or sam didn't talk about him once since the 7.02...that's hurt me a lot...and i hoped to see castiel, just a picture of him (in the death list of dean) or just his name...but nothing....continue to hope to a dean/cass/trenchcoat face to face...and hope maybe one day will can see the third witness...

Amy.

Amy O.o

John or Amy. The reason I think it might have been John is because of the deal John made to save Dean we all know he always felt guilty over the situation so it makes sense to me that he could have been the last witness

100% sure that it is AMY..... I did some review on the episode 4 of season 7,and it came out that when Osiris asked Dean if he want the final witness to show up, a flash back of Dean killing Amy was shown... Dean is afraid that Sam would be mad to him if he (Sam) find out what he (Dean) did to Amy... That's why Dean felt so guilty of what happened to Amy, who is dear to Sam....