Why are some ex girlfriends/boyfriends so vindictive?

Why is my ex girlfriend so vindictive? We met at work (yes, I know, before you say it, don't date people that you meet at work.) Anyway, we dated for about a year then broke up. And initially it was a very peaceful break up. But we still talked, then we started to date again and even began having . But she made it perfectly clear, that she didn't want to get back together. She, herself, told me that she didn't mind if I dated other people. So after awhile, I started to date another girl from work. Well, all hell broke loose.. My ex started saying things about me and my new girlfriend. Then I got mad at her for basically being mad and mean to me, and said things (which I shouldn't have.) Anyway, it's been like 7 months and she even got herself a new boyfriend recently, who she says that she is so madly in love with and might even marry. According to her, she is so "over me", but she is still very cold and completely ignores me. Well, if she is so over me and so in love with her new dude then she can start acting like an adult. At least one would think. So I even tried to make peace with her by writing her an e-mail (I didn't want to talk about it at work) and I apologized and asked if we can be polite, friendly, and cordial to each other. Well, now she goes to my boss and tells him that I'm haring her and she's trying to get me in trouble. Whatever. I knew she had some issues but I never knew that she was just pure crazy. It's not like I ever cheated on her, had a drinking problem, was physically abusive, or left her financially screwed. As a matter of fact, I was always there for her when she needed my help or a shoulder to cry on. So her sheer hatred for me is completely unwarranted. I feel like a total fool having loved someone who can be so horrible. She may very well be the freakin' devil. She is the antichrist. I did so much for her and this is what I get. She was the one that wanted her freedom and I gladly gave it to her. God, I'm so pissed right now. Oh well, guess that's what I deserve.