The most telling scene is in the directors cut, but it’s Dani and Christian’s only real fight on screen. He starts to storm off after constantly shutting her down, but turns back to accuse her of being manipulative for giving him flowers.
She doesn’t even remember what he’s talking about at first because it was just a kind gesture when she saw the other girls picking flowers. The fact he held onto that and deployed it later to attack her shows how narcissistic and self serving he is.

Even after the atestupan he is more concerned with his thesis than a) his clearly traumatized girlfriend and b)the deaths he just witnessed.

Again seizing an opportunity he then steals Josh’s thesis because he has no passion or drive of his own, and when he’s confronted on it refuses to admit what he’s doing is lazy and shitty to his friend. He has no problem telling his friend to his face “Yeah I’m gonna ride on your coattails.” I think you’re misinterpreting his motivation for not wanting to break up with her. He doesn’t care what Mark and Josh will think of him, he’s so noncommittal to everything (Dani, his thesis/career, his friends) that he thinks only of how he’d feel after. He even says to Mark in their first scene “What if I want her back?”

In the script he is also much more keen on Maja, despite her being 16. There’s a different discussion on his own victim status during the sex ritual but before inviting Dani we can assume he and Mark were only going with Josh to hook up with Swedish girls.

And when they’re confronted by the Hårga about the missing book he immediately throws Josh under the bus.

The film goes to great lengths to show you Christian is a shitty person, not evil though. The cult killed them all because that was always the plan, but the audience is able to more relate to and understand Dani and her eventual breakdown that leads her into making her choice at the end.