Dr. Kerr was a modest and extremely popular figure among his 4,800 students and faculty, according to his colleagues here.

He was killed, his friends insist, not for being who he was, but because now that the marines and the American Embassy in Beirut are smothered in security, he was the most vulnerable prominent American in Lebanon and a choice target for militants trying to intimidate Americans into leaving.

''Since he was a little boy, all Malcolm ever really wanted was to be president of A.U.B.,'' said one of his oldest friends, Prof. Edwin T. Prothro, head of the Center for Behavioral Research at the university. ''When the time came, he knew his life could be in danger - he talked about it to me several times - but he took the job anyways because he loved this place and he wanted to build it into something special. In the end he was killed not for who he was or what he did, but for what he symbolized to others who never even knew him.''

According to university officials familiar with the events leading up to the killing, Dr. Kerr left his official residence in the heart of the wooded, 73-acre campus shortly after 8 A.M. and was driven by his chauffeur to his bank in West Beirut. Dr. Kerr had been given a bodyguard by the university soon after he arrived in Lebanon, but he quickly dismissed him, arguing that it was not proper for a university president to go around campus with a bodyguard.

At roughly 9 A.M. Dr. Kerr arrived by car back at the university campus - normally an island of serenity in the chaos that has become Beirut - and paid a quick visit to the office of one of his staff members before heading for College Hall, the main administrative building, which houses his office. Crowded for Registration

At around 9:08 A.M. he walked into the courtyard of College Hall, which was crowded at the time with students registering for courses for the spring term, beginning in February. His assailants undoubtedly found it easy to disappear into such a crowd. According to police sources, one witness thought he saw one of the gunmen rushing out of the building and that he was wearing blue jeans and a leather jacket and carrying a zippered briefcase.

At about 9:09 A.M. Dr. Kerr, a tall, lanky man who was never difficult to pick out in a crowd of his shorter students, entered the elevator at College Hall. Two other males entered the elevator with him, according to a student who was about to join them but decided that there was not enough room and that it would not be right to crowd the university president. It is not clear if the other passengers in the elevator were the killers.