Acts of Omission and Commission and "Mission Accomplished"

What are the ethics of doing versus not-doing? Of quitting versus "staying the course"? If you got a group of people into something, or helped get them into it, and they may or may not be worse off because of it, but you want out, are you justified in counting yourself out? Or do you have an obligation to finish what you started, no matter what? And what if others are telling you to quit? What is the moral calculus for deciding when, for each individual, they say, "I will not be a party to this," as opposed to, "I will try to help salvage this mess," or somewhere in-between?