I believe that its being disloyal while working for your present airline and not giving 100%. It would then be preferable to leave the airline.
If then one finds that the "grass is not green " in the other ailrine, then it is a testament to the old airline that it is indeed a place to work for and that if people are returning to work with them again , the others who are cribbing should perhaps weigh their decsions a bit more dispassionately..
BM