Charla didn't vote Melanie off, technically Beau did because he chose Amanda over Melanie.
Why on earth would Keith make it known what he was going to do?
The reaction from the others would have been just as bad, but he would have had to deal with it in the week leading up to booting Zack too.
The only reason they were shocked is because the originals think they have a right to stay at the hotel.
They thought they were "untouchable" and would just be allowed to pick off the new comers as the chance came available.
Keith finally showed them that they have no more right than anyone else to be there and if they want to stay they better start playing the game.