Actually, from the 1975 third draft in which he was introduced, until the actual shooting script, Ben Kenobi survived to the end of the film. GL knew Luke would still need a Jedi mentor in future films - after all, the hero hadn’t even used his magic sword yet!
The decision to kill Obi-Wan came about during shooting in Tunisia, when GL realized that the Death Star sequences didn’t feel dangerous enough. The original plan had been to convey the brutality of the Imperials by having Leia be badly beaten up (and possibly also with a wardrobe malfunction) after she was tortured.
However, by the fourth draft in January 1976, GL backed away from open violence and nudity on screen, fearing that an R rating would further damage what he saw as the already precarious box-office take from SW.
The result was that the Death Star felt too safe in the script that went before the cameras. GL eventually recognized this and decided to rectify the problem by killing off Obi-Wan in his duel with Vader.
He had to break the news to Alec Guinness during the shooting in Tunisia. As I understand it, Guinness didn’t take it well at first, especially because he thought making such a major plot change during mid-shoot was very unprofessional. In fact Guinness initially threatened to walk off the film, before deciding that the change was an improvement (not least in apparently getting him out of the franchise).