When someone ruins all the good they have ever done!
Solution 1:
If you're nervous around your friend because you just don't know whether or not they may go ballistic on you, you could say that you're "walking on eggshells" when you're around them. And then when they have a blowout, you might have a (temporary) freak out of your own.
Alt: Her angry outbursts "undermine" our good friendship.
Solution 2:
I would say that such people are "self-nullifying". "Nullify" is defined by Merriam-Webster as "to cause (something) to lose its value or to have no effect".