I'm really not sure about this idea that the silencer was not used at all, and that blood was planted in it. If I do dismiss that idea, then the next question is - would Sheila really have put the silencer on the gun? Well she might if she decided to shoot them without alerting any one of them by the noise of the gun. That would mean that she was very calm when she decided to do it, and that doesn't really tie in with the idea of her going "berserk".