The Clinton email scandal is a made up scandal. The Republicans had to start that drumbeat years ago in order to subliminally put it in the public's mind that she is dishonest. It was actually a great sociological experiment to show how suggestible one can be. There is now a sort of group think, where everyone thinks she is dishonest. Her AIDS refusal to testify is just them drawing a line in the sand to show their loyalty.
You aren't supposed to admit to anything in court. That is the first rule in going to court. If you can plead the fifth and not incriminate yourself, why would you? They have nothing to gain and everything to lose by admitting to any knowledge of of anything in a court of law.
The fact that people don't want to testify has more to do with their distrust of law enforcement than it does guilt. In this case in particular, prosecutors and Congress seem like they just won't give up until they find something to use against Clinton, and if they have to take down her aides on unfounded charges they'll do it.
It certainly seems like an aid refusing to testify in the case means that Hillary Clinton is guilty. However, it definitely does not prove anything. This is a high-profile case, and anyone who testifies or is involved in the case in any way is probably going to be scrutinized and judged, so it's understandable why someone wouldn't want to get involved in it.