More spoilers...
From what I saw, his parents were love dovey. After FL died the first time, he made a deal with a "devil", every time she went back in time his life became more and more miserable.
As the price was his own happiness, the surrounding had to get worse and worse, since his happiness would also depend on the people around him. I don't think he thought about his parents when he made the deal, but I can't help but think that the horrible person the father became and the horrible life the mother led, it was all because of him, and believe me, the FL was brought back in time more often than we think, so this is just one of the many terrible lives that his parents have had and the worst part is that they didn't have choice since it's like a curse.
Really? His parents were loveydovey? Before FL got brought back in time for the first time? Somehow, I doubt that. If that were the case, why did the FL remembered that His father was a scumbag in the first place? Not to mention the Mistress that the Father had even before the start of the story. The father's already too strict with Rudrik even when he was young child. Let's say you're telling what will truly happen in the story, that only means FL's memories are memories from her past life, right? ╮( ̄▽ ̄)╭
Not the first time, there were reborn/returned in time many, many times. In the original timeline the very first one (btw heavy spoilers) FL died bc her paternal aunt and uncle were greedy for the Duke title. She game ended herself bc her parents were dead bc of their schemes. ML couldn't save her and came too late, but he made a deal with the devil: FL gets to rebirth and be back in time, the price is his own luck and welfare in life.
And yes those are FL's memories. Another spoiler: O'neil (novel FL) IS the og Dahlia that lived and died those many, many times. She traveled back to the og timeline (don't ask me how I'm just repeating what I've read in spoiler forums) to set things right and figure out why the hell she couldn't die.
But because Dahlia existed twice in the same timeline, the og Dahlia - our FL - inherited O'neil's memories. Those were so heavy and traumatic that they sorted themselves into rofan novel format basically.
It was a teeny tiny bit hinted at at the start of the manhwa when our Dahlia explains how O'neil "came out of nowhere".

I saw spoilers about ML's parents and now i'm really sad for them. Who needs enemies with a son like that? My friend, how do you do this to your family because of a girl?????