Do you believe bystanders have a responsibility to intervene when there is trouble?

While I will not comment on ALL bystanders, I believe that I have a responsibility to intervene when there is trouble.  That is the way I was raised.  I believe that if I have an opportunity to help or to provide assistance to someone, I should.  There are those occasions where not intervening is expected.  Those would be scenarios like when paramedics or law enforcement officials are performing their duties and any intervention on my part would hinder the accomplishment of their job.  But by and large, if I can intervene to help someone out, I will. 

I'd say it all depends on the situation.  If I roll up on an accident, and the cops and firefighters and paramedics are already there, then they've got it under control.  No reason for me to stop, as I'd only be in the way.

If I roll up on someone using a knife to mug someone else, well, let's just say that mugger will be about to have a very bad day.  If they're using a gun, well, that's why I carry a cell phone on my belt.  If I'm off post....I do have the option for concealed carry.  
If I roll up on someone being stupid, and they hurt themselves somehow, sure I'll call the authorities to get them help, but that might be the extent of what I do...

This is a tricky question...  Sometimes when people intervene they make matters worse because they may not know what to do...  or they may get in the way attempting to help someone on the scene where more qualified people are attempting to help...  Since the situation vary, one can never really know whether their aide will help or harm so it is more of a gamble...