Fridge Horror: In Harlan Ellison's original script, Kirk's hesitation and seeming willingness to let Edith Keeler live in spite of history is presented as a grand romantic gesture that even Spock acknowledges. And it seems that way until you remember Spock's line in the televised episode that, if Edith is saved, "millions will die who did not die before". And then it gets worse. With history changed, millions more would've had their existences prevented. And with the Federation erased, even more millions who would be helped by that group would've died, as well. Basically, if Kirk had saved Edith, he'd have become the greatest mass murderer in history.
Fridge / Star Trek S1 E28 "The City on the Edge of Forever"