In the final sequences of Alien, Ripley activates the self destruct sequence of the ship, Nostromo.
After seeing the alien in front of the shuttle entrance, Ripley runs back to the control room and frantically tries to deactivate the self destruct.
The whole sequence can be seen here in pictures.
Why would Ripley make this decision; what's the need for disabling the self-destruct?
Basically she was dead anyway. Nowhere to go and the alien was on to her. So it would just seem to be the best option to destruct the ship and the alien.