I think you're just as guilty as the one that did the crime because if you had the chance to prevent it but you didn't, you're technically okay with it. I don't think there is a level of guilt in that kind of situations. If you witness a murder and you don't call the police, you're okay with it, you could've saved someone's life with just a call but you decided to just watch. Witnessing a crime and not trying to prevent it is like you're doing it as well.