Situations like this have happened before. There are millions of messed up people in the world, and plenty of people to scared to stop them. There have been plenty of brutal murders, including someone in Canada decapitating someone (A nation known for its hospitality, that no one tried to stop. Thats a different story from helping someone after the event. I've never heard of an incident where no one went to actually help the person after. When someone gets shot or stabbed, people usually will call an ambulance and try and help him after the shooter fled the scene.
I'm not sure the decapitation one is a good example of people not attempting to stop something considering it took place at night, on a long distance bus and it happened so quickly that there was no way anybody would have been able to stop it from happening.

In fact, in that example there were several people who attempted to help the victim if I recall correctly but they weren't able to because he had a huge knife, they were able to keep him locked in the bus though and prevented him from getting away.