I don't think Ben ever planned to leave the Death Star. It became a kind of suicide mission once he realized what was happening and that Vader was there, and felt confident enough that Luke would carry on in his place. He figured he'd complete his task of disabling the tractor beam and distracting Vader & the stormtroopers so that the gang could escape.
bkev said:
Didn't Alec Guinness convince Lucas to kill off Obi-Wan because he couldn't stand the dialogue? I remember reading that SOMEWHERE...
I heard the exact opposite- that Guinness was so upset that the script was changed and his character was being killed off that he nearly quit the film. Lucas had to talk him down and explain the reasons that Ben's death would benefit the film, and he finally convinced Guinness to go on.