Thinking about this some more: Maybe it is a bit of both? If it is only that Eva stole Des's boyfriend it doesn't make much sense why Eva hates Des. So maybe Eva stole Des's boyfriend as revenge for Des breaking Eva's heart? Maybe far fetched, but it feels like it is not just one sided.
Why does Eva hate Des?
I don't see Des belittling, demeaning or otherwise bullying Eva.
So it could go like this.
Eva fell in love with Des. They kissed. Maybe continued their relationship. Maybe tried to go
further. Perhaps confessing to Des. Or tried to get more intimate. Des stopped it, claiming she isn't a lesbian. Could even said she was just curious or wanted to try it out. Essentially her feelings weren't as deep as Eva hoped for. Eva was heart broken. They broke up. And things got awkward between them. At least on Eva's side.
Two versions of what happened after:
1.
Eva feels bitter and heart broken. Des finds a (new) boyfriend. Could be shortly after Eva. Des and her boyfriend are all lovey dovey. Eva hates it. "How could she replace me so quickly? She really must not have loved me." Out of revenge she goes and seduces Des' boyfriend. Only that Des really fell for the guy and now hates Eva.
2.
Des finds a new boyfriend. Des is falls head over heels for him. He's probably charming, dashing young lad. Des young and inexperienced and blinded by love. Eva sees it and of course hates it. Is furious. "How could she? And we have barely broken up and she already is all over this guy?!" Only Des doesn't see that her young lover isn't the prince Charming she sees him as. But Eva not blinded by his radiant smile sees that he flirts with other girls, which Des waves away " of course he has a lot of fans, dashing as he is! ". And not just that. Maybe he is just a scoundrel who boozes with his friends after school when he waves good bye to all his fans and not happens to go to a date with his "princess". Maybe he is more handsy with his female "fans" than the occasional smile or kiss on the hand. But Des doesn't (want to) see any of this. When Eva tries to warn her that he's a good for nothing scoundrel and womanizer she dismisses her. One time Eva catches her prince charming giving his attention to one of his fans. Maybe going out of a bar, in a back street. She could go so far as to call him out on it, as he "has a girlfriend, dammit!", but he just shoos her away maybe threatening her not to tell about it to Des. Eva has got enough of it and if Des wouldn't listen she would prove it to her and show what a douchebag he really is. So she meets up with him one day and asks him if he wouldn't go for a drink. Which he after a short hesitation "aren't you her nosy friend? Oh, well, whatever." accepts. Then at the bar after they, well mostly he, had a few drinks she is more forward and applies her feminine charms. He now being tipsy doesn't resist. Not like he would probably anyway. But Eva has to make sure. When he is swooning all over Eva, suddenly Des arrives on the scene at the arranged time of her meeting with her, supposedly to talk about something important. Des cannot believe her eyes her boyfriend making out with her "friend". Her whole world collapses. She makes a scene screaming and shouting at her unfaithful boyfriend. Who under the influence of alcohol only stammers in response. She's got enough. It's over. Her prince isn't so charming anymore. And her friend, no former," lover" who previously declared her undying love for her is now smooching her "prince". She storms off not wanting to hear a word or seeing them ever again. It's over. Not only for Des. Eva feeling like the ground has been pulled from under her feet is shell-shocked, but unlike her friend she feels like in a daze. Everything is getting blurry, images, sounds. She just barely manages to get the drunken sack off of her. She somehow gets a taxi and comes home. Next day in school her friend doesn't want to speak with her and just calls her a whore, slut and accuses her of stealing her boyfriend. Des doesn't let her explain the whole situation. She doesn't want to hear about it. And thinks it's Eva's way of getting back at her for turning her down. Eva is heart broken. Now she truly lost her friend. Let alone any chance to become lovers ever again. For Des she is only a thief and a traitor.
DDoS. Coming soon to a server near you.