
If we use a realistic view, he was simply an aesthetically pleasing easy target. Monsters like him choose people that way, and that's true irl too. He spent time courting him, hoping to lure him and make him his easy, little toy yet the 'toy' actually had relations (based on his delusional view) with someone else. This made him angry, since he sees him as an object he claimed first, and this made him show his true colors(a violent rapist). He's your average predator. Men all around the world literally kill women for saying no, so seeing in fiction a guy raping someone over perceived 'rejection' for seeing someone else isn't that surprising to me. I don't think the scene was random, in fact it felt realistic and chilling.
ok but one thing thats kind of confusing to me is like...? why did he. suddenly get obsessed with him? like was it "love" at first sight? usually theres a reason for crazy protectiveness/obsession/etc but it feels like it genuinely came out of nowhere lmaoo?