Ohh this seems interesting. I feel like the dude with the black hair isn't the one who pushed her off, I suspect the other two or some other yet to be introduced character.
Its weird the guy was invited to her birthday party
it seems they didn't interact together so maybe she had feelings for him but if she did doesnt seem like it now
based off the title it seems the person who killed her did it out of love thinking its if I can't have you no can type deal. Also another reason can't be the black hair guy is he was hoping she would come back again and hoping in that legend.