It is easy to say what one would do. Like if you and Hitler were alone in a room and no repercussions to be followed no matter what you do. In this case, let's say you witness something (hence the title), what would you do? There is a right thing to do and I understand if some people may feel no sympathy for the main character. But if you indulge and accept that flaw, you will see quite a suspensful movie, that works on many levels.
Because apart from a thriller, it is also a social commentary. It is what most people are probably doing and what effect this can have on others. It's not easy doing the right thing, it takes courage. Of course as mentioned not just the main character is flawed, but also the movie itself. But while it is not perfect it is entertaining and might be able to give you food for thought too