Why do people feel sorry for Cassiel, put aside whether he manipulate by that bitch or not, from the pov of Jayne he indeed betrayed her, cruelly at that with no even shred of mercy. Even if he is innocent so what? The feeling of getting betrayed is still hurt.
Try to imagine like your parents or closest person or even yourself died because of a guy accidently trigger a gun, he don't even mean it does this mean he innocent? Or course not right, even if you know it was just an accident I'm sure you all still be angry at him and don't want to involve yourself with him anymore even more if he someone you know.
I know it was an extreme example, but the feeling that Jayna has because of Cassiel I think it close to that.
Edit : typo