Betrayal by Harold Pinter has a unique style which allows the audience to reflect on the effects of bad decisions made years before. He commences his play after the three main characters have betrayed one another. Jerry has betrayed his best friend Rob by sleeping with Emma his wife; likewise Emma, who even after everything (which is actually the beginning of the play) is still married to Robert. The play will move backwards towards Jerry's first drunken, misguided pass at Emma as, as the audience knows, it will not end well for any of them.
Robert himself has also had affairs and, as the audience will discover, has known about Emma and Jerry's affair for years but never cared sufficiently to do anything about it, except, perhaps, to have his own affairs with unnamed characters, suggesting their trivial nature. This all gives insight into the fickle nature of relationships that are not based on trust and loyalty.Those that the characters have betrayed most is themselves although they do not seem to have realized that; even with the benefit of hindsight.
Emma makes herself into something of a victim and could perhaps be described as the protagonist for this reason. Emma is her own worst enemy and sabotages any chances she may have at happiness. Emma could be the protagonist as her story drives the plot. It does not in any way suggest that she is innocent.
Jerry, then, may be the antagonist, because he should never have made a pass at Emma. However, even time could be an antagonist in this play. What is very noticeable and significant is the fact that the beginning really is the end for these three characters. It is their actions at the beginning of married life that signals the end for any true hope of happiness.