So this leads to several theories in my head
1) The MC has timebending powers but he can't control them, so he's unintentionally doing this without meaning to.
2) Some guy playing hero has timebending powers, but they only work in certain ways. So the only thing he can do as a hero is reverse the deaths.
3) Some guy (my guess is white-hair over here) has powers, but doesn't realize it. So everytime someone dies involving the MC, they unintentionally reverse it.
4) The MC got cursed or whatever that would work, meaning that the timebending guy doesn't have to manually activate the reversal and just tied it to the MC killing someone.
5) thats all i got but im sure there's more and probably better theories.