Plus the PLO had come with a new charter in 1968 which was quite violent on a rethorical point of view: Israel was "a fascist and colonialist state" who had "to be destroyed". The Charter accepted the presence of jews in Palestine but only those who were in Palestine before the "Zionist colonisation" ( which is quite vague btw, are we talking about the jews present in Palestine before 1947? 1917? Before 1917?). Those kind of words didn't help to reassure the Israeli government, even if it was only words (Arafat tacitly accepted the idea of a State of Israel in 1974). Add to this the war in 1973 (which wasn't a war to destroy Isreal but to restore Egypt's honor with limited territorial ambitions -Sinai and Golan- but Israel didn't know that back then) and Israel was still feeling threatened in its existence. Thus the nuclear option to feel safe. At least until 1978-1979.