The "Jealous Girl"–is she really so bad?

In most of my works, there’s a shadow character. She’s the “other” girl. The one who is completely jealous of the main character, who strives to be better than her, who is somehow friends with the protagonist–and probably even likes her to an extent–but in reality really just wants her out of the way, permanently. In Within, this lady is Ellie. She’s the best friend to Trinity (the girl who has brain damage). Ellie has a major crush on Trinity’s boyfriend, Zack and her crush only gets stronger after the accident that gives Trinity permanent brain damage.
It’s totally possible that Ellie exists your life. Or maybe, you’re someone else’s Ellie.
I wanted to write from this girl’s perspective, to see what she was feeling. I think we’ve all been in a situation where we’ve had unreciprocated feelings (hello HIGH SCHOOL!). I took those feelings and ran with them for Ellie. Because when you’re infatuated with someone to the point of obsession, you’ll do anything for them, just for them to notice you. And with Ellie, her feelings for Zack were simmering below the surface and only got worse as more opportunities to be alone with Zack came up.
It’s pretty clear from a majority of people who have reviewed the book: Ellie is not a favourite. She tries to steal Zack from Trinity, and Trinity can’t do anything about it, and Zack is all over the place, emotionally.
But you know what? I understand your hatred of her, and I’d hate her if I were Trinity and if I knew what she was feeling/doing too, but we have to look deeper at her motivations here and the actual power of infatuation.
I think it’s easy to be unsympathetic to her unless you’re feeling exactly the same way she is at the moment of reading. Looking back, I’m sure we’ve all done embarrassing things to impress our crushes. We all like to forget those moments, so it makes Ellie even more unbearable to read. We also dislike her because we want desperately for Zack and Trinity’s relationship to work–they are meant to be, says all the rules of romance! Ellie is taking advantage of the only opportunity she has–Trinity’s brain damage–so she can be closer to Zack. Is this wrong? In a lot of ways, yes. But she’s not wrong in her astute observation that Trinity is not able to have the relationship that Zack wants. She has waited a long time for Zack to be available; everyone is just so caught up in the events of the novel that Ellie chooses the wrong time to pursue Zack in that way. Had she waited a few years until Zack could come to terms with Trinity’s changed behaviour–had they known for sure if her condition was permanent–then maybe Ellie would have had more of a chance. But since when is infatuation patient?
Yes, Ellie makes a lot of mistakes. Does she want to hurt Trinity? No, not purposefully. Ellie’s just a girl who is trying to find her way. Just like the rest of us girls.