When I was CPR certified we were told that we were obligated to try to give aid. That meant that we were obligated to call 911.
However, if we found that the pressure of the situation was too much for us to deal with we were asked to not remain on the scene. I'm not sure what the actual reasoning was, but they really didn't want us to remain on the scene if we were not prepared to be part of the solution. This was way back in 1990. I haven't been certified since, so I don't know what the current thinking is.