Some of them will probably shat themselves, others would freeze in fear and some would do both of these things.
"I hope he doesn't have NOED or Ruin!"
"ew wtf are you supposed to be"
Literally exactly what you stated but man, do I miss the Benedict quotes to actually describe at least one persons viewpoint on new killers and survivors.
"Wow thats one ugly m..." *teabag teabag teabag*
Bill and Ash have gone against worse, so they will probably respond like they would with any other Killer. Both Davids would probably do the same.
The others would do exactly as you said they would.
Dwight would keep on doing that all while hiding in a locker.
Nea would try to pet and feed her new found pet.
"Swallow this" *throws a pallet and stuns the Demogorgon* - Ashley J. Williams.
Feng: Whoa, this thing really looks like monsters I shoot at videogames before!
Jake: Iam one with nature. Tho, my spirit is calm, my mind suggest don't pet this animal.
Quentin: My nightmares became worse.
David King: Iam gonna torn up your petals, flowey.
"wait that doesnt look human" *turns to look at sky* "Damn you devs I thought we had a deal"
Meg would be halfway acrosa the arena before anyone else had a chance to react. Dwight would try and keep people together. Jeff gets a great idea for an album cover if he ever gets back to reality. Steve begins a very aggressive search for the nearest baseball bat.