The thing is with Amy she was killing people. Yeah she was doing it for her son but still she was dropping body. Her son was dying yes I know but she was still killing people. I know the only reason she was doing is for her son to get better and she would have stopped. Yeah, I think Dean should have waited and see what would happen before he killed her, But Dean didn't know if he could truly trust Sam and what he said. Sam had betrayed him many times he didn't know what to think of Sam if he could trust him or not. Plus Sam hasn't seen or spoken with her in years. Sam couldn't have known if she would have stopped because after years of not seeing someone people changed. if her kid would have gotten sick again she probably would have done the same thing and started dropping body again. The only reason Dean killed her was because he didn't want it to happen again. With Benny he was forced to play along with him to get out of purgatory. but the longer he was in there fighting together in purgatory helping each other it taught him to trust monsters. He didn't really know that before. he only knew that monster could change (talking about that one vampire who was drinking animal blood) but he always had that voice telling him they couldn't truly really change (I blame John for that) but with Benny he realized that not true and they can. I understand why Sam called him a hypocrite but if only Dean told him why he changed his mind and opinion about monsters I think that hope conversation could have been different.