The dilemma of "Lift Us Up" is especially hard to face.
Imagine you're in "City on the Edge of Forever". Someone with infallible knowledge points out a young woman on the street and tells you "Edith Keeler has to die in a car accident to stop the Nazis from winning World War 2."
Pretty bad. But all you have to do is nothing.
Now let's say the same someone hands you a .38 and says, "Edith Keeler has to be murdered in the street to stop the Nazis from winning World War 2."
Little harder, isn't it?
Now let's suppose the same someone says, "sorry, that isn't Edith Keeler. That's her over there." They point to the 9-year-old standing next to the woman.
Now it's horrible. But your infallible source of information isn't done. He takes away the gun and hands you a pair of pliers. The implication is clear.
So at what point do you decide it's not worth it?