
The ML wasn't the one who killed MC, it was that annoying girl Julianne who had her chamberlain (a sorcerer who is insane for Julianne) use a spell to disguise herself as the ML. After slashing MC with a sword, MC calls her out that there's no way it is actually the ML because ML would never harm her or speak rudely to her, so Julianne reveals her identity to MC and tells her she will attack (the spoiler said attack but i assume attack as in getting him to do it with her?) the ML disguised as MC.
Julianne tried to seduce the ML but he instantly knew it wasn't the MC and tries to cut her down but the chamberlain protected her. The chamberlain mentioned the ML's sword which is able to grant the owner's wish using blood which shocked the ML because no one but their family was supposed to know. Something happened afterwards (not entirely sure) but the chamberlain stabs Julianne and ML quickly ran to the room where mc was and he sees her on the ground. ML picks up her body where MC is barely alive and is glad nothing happened to him. She says "Thank goodness" before passing away. He held her body for hours until sunrise. Eventually the ML cuts MC's hand while apologizing to her and slits his own throat right before the prince and other people were able to get into the room.
ML was the one who sent them both back to the past but because of his wish he had a constraint placed on him. Example: He cannot talk about him regressing and etc. He truly loved mc because they met when they were younger and the reason why he glares at her whenever he sees her is because that's his yandere stare for her lol.
The reason why MC only remembers up to the part when the disguised Julianne slashes her is because a part of MC's will mixed together with ML's wish when the sword cut MC's hand. She believes that if she didn't love the ML he would be happier so the sword tried to do that by not letting her remember the whole situation.
There's a more in depth spoiler about the novel and more parts about the rest story on novelupdates (like what happened to her stupid prince ex fiancee ) if anyone is interested
Like some of the other comments mentioned, I think the ML likes the MC in the original story but because she died, he tried mending his broken heart with her older brother as they look similar.