Can People actually change or do they just pretend they do?

ok so my ex girlfriend and her mate was slagging off my family pretty bad saying "i hope they die" etc. -.- (but my ex insists it was all her friend) anyway, but she always used to try and annoy me by being nice and loving at first but then just before she goes she either starts and arguement or purposely takes something the wrong way to make me get angry :@ �also, when i was dating her (from other people's opinions including my mum, dad and sister) i was a complete ****, ditching my mates for her and she kinda treat me like **** anyway she used to say get that for me and just played with my head pretty much on a regular basis and she used to have a go at my l mates because she thought i was cheating on her with them -.-. i was just wondering.... 'can people change?' i currently am dating her again but i don't know if i should carry on doing this. i just wanted to know can people change from what they were, if so, how can i know if this is going to happen because she's pretty much the same as before.thanks