Character wise, Shadow Shy actualy makes alot more sense than people seem to think. Shadow was also kind gentle and caring about life and the world. Until the world showed up, smacked him around, killed his famill, destroyed his home, ruined his life, and then imprisoned him for years only to be awoken by a power hungry mad man who did nothing but prove to Shadow that humans were nothing but scum.  
the good news is… he got better.  
I can TOTALLY see Fluttershy undergoing that same path and route. Hell you kinda see it a bit in the comics where she goes totally evil thanks to dark magic. [she also got better]

is will work more with a other Twilight but from a other dimension but fluttershy be Shadow? is not work and is why Sonic and Shadow do this face!

Sonic: the girls do some Role-play?  
Shadow: Yeah, Role-play…and is not work by the their look!

@Red-Supernova  
The video is based in that synthetic voice generator and only Twilight and Fluttershy are available on it.
 
Hence why Fluttershy is used.

Shouldn’t either Sunset or Starlight be in Shadow’s place instead?

Yeah, those lines didn’t even sound good coming from Sonic and Shadow.