Rejecting a PhD offer after giving promise to the supervisor to work with him 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.
You should have been cautious with your word, especially since your prospective supervisor told you about prior disappointment. Being noncommittal would have been more unpleasant, but more honest and giving you more leeway. 

On the other hand, the supervisor might have put you under pressure  and you caved (depending when s/he told you the story with the other student). 

I do not see a "good" outcome unless you are lucky to get turned down immediately, so that you immediately know to go for your original offer. That not being the case, the other outcomes are bad, even dire (waiting for response, being rejected, the superviser then realising that s/he was only a substitute).

I do not know you, but my hypothesis would be that you were inexperienced; but the damage is done. So, all that I can recommend is that you should consider whether, ignoring for a moment the quality of the university (which is the reason you want to leave), you would like to work with that supervisor and on the topics s/he represents, all other aspects of the positions on offer being equivalent. Reflecting on that may help you make up your mind.