Rabin was allegedly the best secured head of state in the world. The thought that a civilian could enter the “sterile area” surrounding Rabin was unbelievable.
Later, it became known that the assassin, Yigal Amir, was targeted by the Shin Beth months before that and that the Shin Beth trained him in the use of arms. Avishai Raviv, a Shin Beth employee, was placed next to him. They claimed he was only an information gathering agent. Yigal Amir, a former Yeshiva student studying at the religious Bar Ilan University, killed Prime Minister Rabin with the blessing of his Pharisaic rabbi. He moved from one rabbi to another until he found one who agreed that the killing would save many Jewish lives; and thus – the religious authority claimed – it was justified.
Raviv knew that and reported Yigal Amir's plan to assassinate Yitzhak Rabin, based on “din rodef,” law of the pursuer, which rabbis interpret as allowing the killing of another Jew to prevent him from handing Jewish land over to non-Jews.
Despite that, the Shin Beth did not stop Amir.