If you want to be able to change a bad decision in upcoming action-RPG Vampyr, your only recourse is to start the whole game over. The game will only have one save slot. Evidently Dontnod really wants you to stick with your choices, no matter how much you regret them.
Dontnod has said repeatedly that your choices in Vampyr matter. Protagonist Reid can investigate the lives of citizens to determine which he will sacrifice and which he will save. Whom he kills affects the overall storyline, as each NPC have individual stories and unique impacts upon their districts. If Reid kills too many of them in one district, he could destroy the district entirely. Obviously, your choices are not to be made lightly.
Another potential problem with one save slot is the risk of game crashes, bugs, and other issues that could corrupt a save file. This is part of the brilliance behind post-launch patches; these patches fix any unforeseen problems or issues that did not occur in Q&A. The real world is a big place, and some issues simply cannot be predicted, especially when it comes to video games. However, the game team knows such issues can arise, and they’re “working hard to make sure no technical issue will obstruct your playthrough.”

Yes, you can play with keyboard and mouse on PC. Yes, you only have one save. We’re working hard to make sure no technical issue will obstruct your playthrough.
? Vampyr Game (@VampyrGame) May 4, 2018

Well my fears are allayed....