And let's not forget that even though we didn't see the changed timeline McCoy accidentally created in "The City on the Edge of Forever" one doesn't have to work very hard to picture an Earth where Hitler used the A-bomb and intercontinental rockets to win World War II and the global darkness and tyranny that soon followed. Hundreds of millions if not billions could have died during the three centuries of evil that followed Edith Keeler's successful efforts to keep the United States out of the war.
Spock's line to Kirk next to the staircase remains one of the most chillingly effective lines of dialogue ever delivered in the Star Trek franchise. "Save her. Do as your heart tells you to do. And millions will die who did not die before."
Pretty dark stuff for '60s television.
And Harlan Ellison has the gall to keep bitching about why his original version of that episode was unfilmable?

(For those not in the know: In that version, Kirk actually tries to save Edith's life, knowing full well what kind of timeline this would lead to. How could anyone possibly trust him after that? There's not a chance in hell that could ever be filmed.)