Just going off the question in the title, no, because then I would consider myself a murderer.
Having the opportunity to save someone while risking your own life in the process is not an easy choice to make. I don’t think someone should be charged with murder if they witness the crime, but if they are aware of a plan that is going to be executed, I think that constitues some form of ‘conspiracy to commit a crime.’
There isn’t always something that can be done. If someone has a gun and you are unarmed, what exactly would you be able to do?
Personally, when I saw an assault/murder in progress, I called 911. That was the extent of what could be done. Unfortunately, I did watch that man die, but I find myself in no way responsible.