Is it normal that i want to kill my best friends friend?

you don't want to kill her, but you do want to stab her in the back. you know it's exactly the same thing right?

Your relationship with Avery doesn't seem to be one of a healthy nature. It's her choice to make up her mind about the company she keeps. Suppress your frustration and move on, show Avery that you can be on your own be social with other girls and boys.

I think it is normal to feel hatred towards others who try taking someone away, or treats someone you love badly. Can't tell you how many times I wanted to kill someone. Thing is, in life we can't place our happiness in other people, because no matter how good, or how close the person is to you, we are not perfect and people will disappoint you. Now, I know right now this is all you are mostly focused on, but as we get older we will realize how we are all better off just not to be bothered by things like that. Because no matter what age a person is, even when you get older you will meet people who will remind you back in your school days, and not always in the best way either. The sooner you can be your own alpha and know how to stand on your own without needing someone around to fill your happiness, the better it is going to feel that you can rule the world alone, but you don't have to exactly be alone in order to being able to do that. It doesn't exactly mean you have to be numb, you will still feel, it will just be easier to deal with.

Just don't be the person who wants to kill for someone's company. Be the person someone would want to kill another for your company. Even though that can be terrible too, but at least it won't be you behind bars. Avery made the choice, so take that as you are above both Avery, and Leah. That is the thing about close friends, sometimes not all of them stay close friends. And sometimes the one who feels left behind, instead of seeing their own potential of being more, they get mad enough to kill, then get arrested and wasted a lot of years being locked up and heavily medicated. So forget Avery, forget Leah. Be your own best alpha. Make some new friends yourself. Because no matter what others may say, you can have more than just one best friend. Give it some time. Try to focus on other things, if you like music for example, try to explore new music using youtube or something. Back in my late teens in my more troubled rebel years, Simple plan, and My chemical Romance was really there to help me. But those might not be your kind of music, and that is ok. You just got to be able to got you. In other words, be by your own side first before anyone else. Because on down days, and you will have them, everyone does at some point, it is easier to get over it if you stand by your own side. Be well. Hope this helps.

You don't have a mental disorder, it's called jealousy ..

It's one thing to wish someone you hate would drop drop, but it would be something entirely different to actually kill her of course.

... and you know this.