Is it was legal to use a video game emulator if I already owned a purchased copy of the game?

Do you mean a video game console emulator? If this is what you mean, yes, it is legal. It might be illegal if the console emulator is unauthorized, but that is more a matter for the makers of the original model to assert their patent or copyright claims. By the time you have it at home, it should not be your problem. How else are you going to play your favorite old games on your new system?