The pterodactyl effect
Minor actions having huge unforeseen consequences is a staple of time-travel stories. Pull on a thread, and you unravel the tapestry. "You go back before World War II and kill Hitler, and maybe you make everything worse," says Mercer, taking the opposing view of the conventional wisdom of the classic premise. In his mind, you just don’t make that supposedly obvious choice.
I enjoyed hearing that, because the "kill Hitler" time-travel scenario always sounds good on paper, but you truly have no idea what the downstream ramifications of doing that would be. If you kill Hitler and change the course of history, the results could be counter-intuitively catastrophic. Maybe now the Cuban Missile Crisis ends in global nuclear annihilation. At the very least, you are all but ensuring you will not be born. Maybe I’m selfish and we’re historically far enough removed from World War II to distance ourselves from that pain, but I happen to like the world as it exists today, and doing something as universe-altering as killing Hitler could mean I’m not only erasing my own existence in favor of that alternate reality, but creating a potentially worse one.