When I went for my very first PhD interview by a potential supervisor, I hadn't applied for other universities. I promised that supervisor that I would definitely work under his supervisor if I was given an opportunity. He told me that he had a bad experience of a student replacing him with another supervisor after 1 yr of phd course. So he repeatedly ensured that I would work with him. But now I have been offered a place there. Before getting the offer, I applied for two better universities and I have been waiting for the results of these two uni and I need to decide whether to accept or reject the offer by the first uni before I have known the results of the other two.
Please advise me what I should do. Is it unethical if I turn down the offer after giving such a promise? I know it is risky to wait for the other results if I will be rejected in other two.