@NOUN what? who is going to kill who? the young girl that in the other life was supposed to be a villain after a great catastrophe was killed by the original MC. She did not interfered nor was part of the "timeline" of anyone prior to get traumatic change that made her decide to go down the path of "kill them all."
I think they are doing the whole let's save her cuz she is still innocent and good. Meanwhile the other villain the new MC is countering is already ruined. He wouldn't follow the new MC. How is he going to save him if he knows that he won't be of any use. The difference is that this MC is good and want allies. The original MC is a sociopath. He would have recruited him, too but the male villain wanted to be the leader and kill whoever was in his way. Remember in the new MC thought, it showed the background of the male villain.
I am sure the new MC would have saved him as he did with the girl if he was good. I saw other mangas/manhuas that, finally, the MC slap, kill, or mutilate females because they were bad.