It's made fairly clear in the novel that the fiance is being manipulated by some force making him behave that way. Possibly fate. There was an entire chapter from his POV explaining this.
As the kind of sick, twisted person who reads fafoo novels for the emotionally masochistic thrill, I deeply loved this novel. It took me right back to when I read Pain, Pain, Go Away (both featuring a time travel mechanic. Funny coincidence) and I thought to myself, "What did this person ever do to deserve this?" But by the end, I realized that there was nothing. There is no rationalization for suffering.
The way that Ilya reaches a self-actualization moment in her acceptance of fate is honestly one of the most uplifting things I've ever read. Sometimes, you can't change fate, you just have to find something worth living for within that cruel fate.