Why are girls so vindictive and backstabbing.oh and very arrogant?

sorry about this question being so long but i need to get my frustration out... this girl who is meant to be a friend keeps turning everyone against me..and going off and making me feel like crap then says everyone else is really immature for being bitchy when she's the one who is being the most bitchy! She is only 4'10', ginger and looks like a witch but loves herself so much, and struts about thinking there's no one on this earth who would ever measure up to her. She is nasty and always throws a tantrum if i argue back with her, like when i said i wanted to be a doctor or forensic scientist she said a beautician's course if the same, and they study the same sort of thing, i was like uhh no they don't, they are completely different and a beautician is much easier. She lost her temper and shouted at me saying that they are basically the same for studying, so don't argue her point cause she 'knows she's right' WHY is she so arrogant?! She's really annoying me. She takes like a million (literally loads) of photos of herself constantly even though she isn't even good looking! everywhere she goes the camera's out and she's pouting and posing..everywhere we go she's too busy looking at everyone thinking all the guys are checking her out, when no one can even see her.she's being really irritating and i feel like i'm about to explode if i see one more picture of her. What can i do bout this? Am i overreacting? thanks to anyone who took time to read this and answer..x