Why does this girl do this? PLEASEEEE ANSWEERRR!!!!?

This girl once initiated chat with me on fb.And said she liked me and then said she was just pretending.And then she would tease me and make up excuses to call me and tease me or just to say hi.When she was just saying "Hi" she didn't act mean when she said it.She just said it as if she was bored and she needed to have someone pay attention to her.She would follow me and stare at me.She stalked me at times.And she wouldn't stop annoying me.She was such a nuisance! When I threatened to tell someone that she wouldn't leave me alone and she was stalking me,she got so scared and was begging me please not to.And that she was sorry.And when she did that she would later on go on by using a new strategy.She and her friends would come up to me and just stare at me.And once they asked "Who ya textin'!?" and they would laugh and giggle.She doesn't bother me much anymore anymore.But when she starts talking to me she never shuts up.I feel bad because she doesn't have many friends and people say she has skin disease.I just can't understand what she wanted with me.And she is kind of cute because she's short.You can like fit her in your pocket.Btw I'm a junior and she's a sophomore.And her friend who's a boy starts saying hi to me now.And I'm nice so I say hey back.She's like so weird.I once got to the point to where I said I wish she died-since she couldn't leave me alone.And I felt so bad.I regret it.I mean she stopped everything completely aside from her friends saying hello to me now and then.I asked her to please stop annoying me.And she said it was fun.Then she got scared because I threatened to tell on her.And she doesn't talk much to me now since I said I wish she died.And what I exactly said was horrible and I feel so bad about it.I mean I made a mistake.No one deserves what I said.And I never meant it.It was the meanest thing I've ever said.And I'm not a mean guy.I said I wish she died in the holocaust.And she stopped annoying me 95% when I said that.I still feel horrible about it.I wish I could go back in time and change what I said.But I can't :(