Personally I doubt Luffy would forgive any enemy masterminds who had no qualms hurting those they should have been protecting. At least that seems to be Luffy's main interest in his own role as a captain: He places his friends (crew members) above everything else. Even finding One Piece.

Thus, I doubt he would forgive either Crocodile or Arlong. Releasing Crocodile now would be a very risky tactical choice anyway, even for a smarter man than Luffy, because Crocodile would go out of his way to make sure Luffy wouldn't end up escaping the prison.