The Bad Future could have been avoided by convincing Edith they were time travelers and that she shouldn't form a peace movement, and/or by taking her through the portal to the future so that she disappeared from the timeline and might as well as have been dead (like what happened in Star Trek IV: The Voyage Home).
Then again, the Guardian of Forever might not have agreed with that second idea.
Telling Edith not to form a peace movement would have unforeseen side effects. Besides, would she actually listen due to her convictions? For comparison: imagine if a time traveler told Kirk to drop out of Starfleet and be a farmer in Iowa, then left.
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.
