It’s not that complicated, relax :). You have to point out what could arise which means that it doesn’t have to, in the end. Every researcher has biases and, for example, if you yourself have a history of overeating or feel negatively towards those who do, studying them objectively is probably not going to happen and that’s an ethical dilemma. If you think smokers should have no rights, studying them poses an ethical dilemma because you should feel neutral, etc.