I broke up with my girlfriend, now she's threatening to kill me and then herself?

She was my first girl friend and I was her first. We were best friends for 5 years before. We're going to be seniors in high school. She has always been really negative and pessimistic and I just couldn't handle it anymore... I lost the feelings I used to feel so strongly for her. She didn't show me a lot of affection and was always thinking about the future and getting sad about it because she thought we wouldn't last. She would always worry about things to the point where we wouldn't be able to enjoy our time together. Some times she was rude and mean to me. Also clingy. I didn't want to hurt her. I love our friendship and I love her but I was not in love with her so yesterday I ended it... I get a call from a close friend of mine and he told me she called him and was threatening to kill me and then herself and that she was cutting herself. His mom is a therapist so he gave her the phone and she calmed her down and is going to talk to her parents. Her dog just died too. I didn't mean for her life to be ruined and I wanted to stay friends... I really did. I just wanted to rebuild the friendship we had and I even said that we can start again from there... I did love her... My friend told me not to make any contact whatsoever with her. To forget about her. I'm taking his advice but it's hard. I care about her more than anyone... I just couldn't be with her romantically. What do I do? I feel terrible.