If I see it and do nothing I AM morally responsible.
I have heard things many times and although I might not be able to intervene myself I have called the police and let them know the situation was urgent. The police always came within a few minutes and one case put a man under arrest immediately.
In all cases there was the threat of violence. I am not trained for that so called the people who were trained to deal with someone who was potentially violent.
Although I have interceded once before the police arrived to get an innocent bystander out of a situation that could have resulted in him being attacked. What amazed me was that the staff who deal with those people on a regular basis and knew both men, had no idea how to rescue the innocent man without endangering a dozen other people. But it ended well once the police finally arrived.
Our moral responsibility has nothing to do with any god. it has to do with my personal decision to help others when I can. That doesn't even have anything to do with how I might feel if I did nothing and a person got injured.
It is a matter of doing what is right and moral and ethical in the immediate moment without thinking or considering the cost