I think they would all have different reactions. Applejack wouldn't like that Fluttershy hid something like that from them all this time because of the honesty aspect, but she's practical enough to understand why she did and would ultimately have no problem with it.
Twilight would be really bothered by the realization that she doesn't know anything about gun policy and the contradiction that Fluttershy did something highly illegal but it worked out and seems to have been the right thing to do. She would probably completely dissociate from the personal side of it and just immerse herself in studying gun control and self defense theory until she figured out how she feels about the whole thing.
Dash would either be really impressed and think it was really awesome that Fluttershy was a badass and did something to save them all when no one else could, or she would be weirded out by the whole thing because she never thought Fluttershy would be capable of something like that. She would pretend everything was fine but would be noticeably more awkward around her afterwards.
Rarity would be bothered by it the most because she would feel the whole thing was so uncouth and unpleasant, and surely it all could have just been avoided if people just didn't have guns in the first place? But she would know not to say that to Fluttershy's face. And it would also be hard for Rarity to enjoy hanging out with Fluttershy and going to the spa and whatnot after having seen her kill someone. Their relationship just wouldn't be the same.
As the other Anon said Sunset would be the most levelheaded and would get sick of how weird and judgmental everyone is being about it and would call them out and explain why it's wrong for them to let any of this effect their friendship.
Pinkie would just not give a shit at all beyond being glad everyone was safe and would make off-color jokes about guns and Fluttershy being a killer all the time that make everyone else uncomfortable.