Suppose you have a nice and kind friend who helps you whenever possible, but sometimes they say something to you so angrily and sarcastically that you forget about all their kindness and help, because you feel so annoyed and upset by this sudden change in their behavior or intonation!
(They are not two-faced at all, they just lose their temper badly, especially when you reject their requests or demand, and you feel so upset that decide never ask nor accept their help any more!, actually you feel this behavior neutralizes(?) their past kindness to you, in that moment. However, since this behavior is part of their nature, this doesn't necessarily result in your friendship coming to end, since they are basically kind, they might apologize you later and you forgive them, but this story will be repeated again and again!)
An American friend told me I can say:
Sometimes, my friend ruins all the good she has ever done by losing her temper or acting weird.
What is the most appropriate adjective, expression, idiom, or proverb for describing these people when they act like this? (something that means someone neutralize/ undo(?)or ruin their good doing that easily )
In my country we use a proverb: someone acts like a dairy cow that once in a while kicks over the bucket of milk — just after her milking is finished — and causes all the milk be spilt and wasted. (While the given cow produces the greatest amount of milk among the other cows in someone's farm!)