It honestly would depend on the situation. If I was put into the situation where I knew a stranger was at risk, I would put myself in front of harms way or at least do "something" about it.
The truth is however, we're human beings and we all make mistakes. So the only answer I'd have to that would be a 50/50 chance depending on what type of personality the individual has.
I personally, would risk my life if the circumstances meant I had to do so.