Suppose you are a scientist, and suppose you get an email asking you to peer-review a paper. This email contains the abstract of the paper, so that you can assess whether you are qualified to review the paper. Reading the abstract, you find that you are qualified, as the topic is one you know well. You also notice that you are deeply skeptical of the argument being made, and that you are very likely to recommend that the article not be published. What is your professional duty? Should you try to read the article with an open mind, despite your misgivings, or should you simply decline to review it out of fear of being biased?
In this situation, I did the former, reasoning that if we only review article we are sympathetic to, many terrible articles will be published simply because skeptical reviewers eliminated themselves, and some good articles with unpopular claims will be rejected for lack of qualified reviewers. I read the article, found it irreparably flawed in several major respects, and suggested that the journal reject it. I looked hard for nice things to say about it and didn't find much. While I'm confident my review was accurate, I'm glad these things are anonymous.