When the crew learn that the ship is set to automatically return to Earth they are clearly disturbed by this. What is Earth like at this time in the Alien story? What has happened to Earth to make them so afraid of returning there?
In the film, Johner says, "Earth? I'm not going to that fucking slum."
A deleted final scene actually shows us the state of the planet - and it doesn't look pretty. It would appear that a cataclysmic event has left the planet a barren wasteland.
I, too, would be reluctant to return to Earth.
I believe that they blew up the ship ultimately to stop the possibility of an alien infestation from occurring on earth. This has always been Ripley's mission in the previous films...therefore her Human half took over in the end.
My assumption was that although they didn't like earth they didn't want to see it overrun by the alien species.
They were afraid of returning there because the aliens would be unstoppable.