I'm confused. I thought the black hair was behind all this.
But why is black hair's hand injured? Is it because of the curse or was the black hair got injured by one of the the victim by accident?
If you look at the black-haired girl's wrist at the end, you can see that the bandages are falling off. I think this is supposed to signify that she is faking her injury and that she is actually the one behind all the misfortune. And the reason she's doing all this is so that she can have her friend all to herself.
Or maybe she got hurt while setting up whatever misfortune befell the girl who lend Yuuka that eraser?
Definitely needs that yandere tag.
But that would spoil the mystery... Granted, it's not a huge mystery, but in a short work like this, I'd say it's a spoiler.