Game Concept
Like GTA with playstyle, you are thrown into a free-roaming, pre-genned world, after you are shown a video of the character getting cursed etc.
Your curse entails you to kill people or bad things happen to everyone around you, i.e. a small cyclone/windstorm occurs and people get electrocuted. Someone nearby might slip and land fatally, causing death, stuff like that. You must kill people who have been saved, because they were fated to die. 
You do have free will, you may choose to kill everyone, but you get marks for how good/bad someone was. The NPCs will be thousands worth, each with their own good/bad, with variables that change that as you play. Butterfly effects, in a way. You may cause someone to seek revenge or their sanity may break, you may be a sign to someone who has a change of heart.
If you kill bad people, you get good marks, if you kill a bad person who is turned good, you get bad marks. If you don't give someone a chance and kill them as a fight breaks between you and them, you lose a chance and a random mark is given depending on when you kill them during the fight and whether or not they were pretending to mean what they were saying.

This will require a fuckton of work, but all in all, when it's done, it will be a unique and fun concept that will bring a new experience every single time you play, no one playthrough will ever be the same.

To finish the game, you must defeat your altenate universe self, if you choose to find a curse reversal, you will have to fight the you that didn't, and vice versa.

(May add in time travel with time travel ideas I have, which causes you to have an alternate universe self)